This Instructable will cover everything you need to know to get started with your own backyard movie theater. This is a great way to get the family and neighbors together in the evenings. I refrained from calling it a "Drive-In" since I don't want anyone parking on my lawn. Here's the summary of what you need.

1. Projector
2. Audio system
3. Screen
4. DVD player or laptop
5. Audience, snacks, chairs, blankets, etc.

This Instructable will be entered in the Dadcando so please rate it and vote if you like it.
EDIT: Thanks to everyone who voted for me in the contest. Didn't make it as a finalist,but I was very happy to get as many votes as I did!

Step 1: The Projector

The projector is the heart of this system and it's something you can't really improvise. Well, you can, but that's another instructable. If you can't borrow a projector from work, you'll need to purchase one.

New projectors start at about $400 and go up from there (way up.) Used projectors can be had for $200 or so if you're vigilant on craigslist or ebay. Beware of old projectors with high hours or that need new lamps. The lamp in the projector will eventually need replace - usually in 1000-4000 hours. Lamps are expensive, usually $200 and up. Most projectors have an hour meter somewhere in the menu. If the seller won't tell you how many hours, be very careful about buying, especially if the price is too good to be true.

Brightness and contrast are the main things to consider. My projector is 1200 lumens and 350:1 contrast ratio. In Eco-Mode, the brightness drops to 1080 lumens, but the lamp life doubles. The difference is so minimal, I only run it on Eco-Mode. The 350:1 contrast is pretty low. If I had my druthers, I'd like something up around 1000:1 and 2000+ lumens. However, everything is relative. If you're the first kid on your block to do this, ANY projector is gonna look awesome.

There are 2 types of digital projectors - LCD and DLP. LCD uses red, green and blue, LCD panels to make the different colors, just like your old-timey tube TV. DLP projectors use a rotating disc to reflect the light off a chip to make the red/green/blue combination. Each has it's advantages. In general LCD has better color, but worse contrast than DLP. Contrast is important when outside because it lets the black look darker. Some older DLP models would produce a rainbow effect that some people could see. That's mostly fixed on newer models. Google "LCD vs DLP" if you want to read more.

Lamp life and replacement cost is a consideration when choosing a projector, but even at 1000 hours of life, you could use it for 4 hours a week for 5 years. If you're doing that much movie viewing, good for you!

Best resource I've found for checking out the specs on used projectors is www.projectorcentral.com. They list most projectors and give everything you wanted to know and more. Here's what I'm using. Not exactly top of the line.
http://www.projectorcentral.com/Sharp-Notevision_PG-B10S.htm

Step 2: Audio System

I went old, old school on this. This amplifier is 1970's vintage and it does the job. Grab handles and vu meters, how can you go wrong? This was a freebie from someone's cleanup day. I use a laptop as my video player, so I use a 1/8" (3.5mm) to 2 RCA adapter cable. If you're using a DVD player, the standard RCA jack patch cord will work.

For the speakers I have my big old Technics 12" 3-way cabinet speakers. These have great sound, but are bulky and kinda heavy. In a pinch you could use a set of computer speakers or a surround sound system. Some projectors have built in speakers, but those are only really useful for presentations, not big time movie audio.

For the cables I made my own monster cables. Remember the time you hacked your 100' extension cord with the hedge clippers? No? uhh, I never did that either. Well, raid your neighbors trash when he does. Cut the ends off, cut the cord in half and you have two 50' heavy duty outdoor rated speaker wires. Tin the ends with solder if you wanna get really fancy.

Step 3: The Screen

For the screen you have many options. I was lucky enough to scrounge a roll of heavy white vinyl app. 5' wide. I patched it together and ended up with a screen 12' wide by 7' tall. Superglue works great on vinyl, so no sewing or taping was needed. Check your local sign shop and they might be able to supply some.

We have a roll-out awning on the back patio, so that gave me an easy way to hang it. I support it at the top corners and pull it taut to the sides. At the bottom corners, I have two strings that attach to hooks on the back of the speakers.

Other options.
The old standard is the white bedsheet. Sheets work fine, but let a bit of light through so they won't be as bright. Also, they're only so big, so joining multiple sheets together requires some sewing.

Plain white shower curtain liner. Two or more of these can be taped together with clear tape and they work very well. My first few movies were done with these. Also, they already have holes for hanging them. Don't try to pull them too taut or they will rip. They're less than $5 each, so this is an affordable way to get your feet wet.

If you have an old slide projector screen, that can be used, but they're usually only about 4' across. Again, if you're the first one to do it, it'll seem huge, but once you go big, you can't go back!

For the bigger budgets, there are inflatable screens. These are nice when you don't have any structure to hang your screen on, but they tend to be noisy as the fan has to run continuously. The blower is like the kids bounce-house fan, so that may tend to get annoying.

For nearly any of the fabric screens, they're going to get some wrinkles from being pulled by the corners and can ripple in the breeze. This isn't really noticeable most of the time. One solution is to use something rigid, like painted plywood. Drawback to this is storage and weight.

The shot with the webpage showing was taken just as the sun was setting. Not quite dark enough for movie viewing, but it gives you an idea of the size. In this example, the screen was at the usual TV 4:3 ratio, so the top and bottom were cut off. I always use widescreen/letterbox format for the movies, so I use the full 12' width, but it doesn't cut off the top or bottom.

Step 4: Media Player

This one is pretty simple. You need something that can play a movie and output it to your projector. Some projectors only have computer monitor type connectors. Most have that, S-video and/or RCA video in. Use whatever works best for you.

I like to use the laptop because I can easily make a playlist of previews and cartoons to make it more like a "real" theater. Search youtube for "drive-in intermission" and you'll find some of the old 50's and 60's stuff. Those are especially fun when you have some older folks in the audience. www.movie-list.com has a lot of trailers available for download as well. Check their archive for old school stuff.

Step 5: The Audience Is Listening

That's the crux of it. Get some chairs and blankets, pop some corn, and movie time.

Since it is a backyard and kids are gonna be running around before the show, I stress the number one rule. "No playing around the projector." Better to say that 10 times too often instead of one time too few. No projector means no movie night!

Step 6: When It's Over

When it's all over, it needs to be taken down. I usually wait until morning to take the screen down so it has time to dry out (it gets damp at night sometimes). As much as I'd like to put off the rest of the work until the next day, I make a point to get all the electronics put away when the show is over.

When you turn off the projector, let the fan run until it cools down completely before unplugging it or moving it. Remember, that bulb is expensive, treat it well.

And when summer is over the fun doesn't need to end. When the weather gets cold, we bring it inside. Our living room has a 6' wide picture window and vinyl blind that makes for a perfect built in screen. Follow the same rules, but the screen's just not so big.

Happy Viewing!