I got a bike messenger bag at some street faire, and it's so comfortable!
I've decided to make one from this nice dotted canvas fabric, and I love it! It would probably work well with any heavy weight fabric, or you could use a light weight fabric, but make it double sided - possibly with a different color inside. You can prepare a strap from the same fabric, or use a pre-made one. While the second option is much faster and easier, preparing your own strap might be very pretty for some fabrics.

Step 1: Marking the Fabric

Making this bag is indeed very easy and very quick. I used the original bag's pattern. You can use these measurements:
Strap: 98 cm (you'd better measure yourself to make sure it's comfortable)
Bag: 32x37 cm. Leave 2-3 cm on the sides for seem allowances, and about 3 cm on the top for the hem.
Due to the fabric's pattern, I've decided to mark it on the right side, with a light colored chalk. This way I was able to make sure the dots will come out as I'd like them to.

Step 2: Preparing the Strap

If you've decide to prepare the strap yourself, then it's much better if you pin the fabric first, especially if you want the fabric to keep a specific pattern, like I did here. But even when there's no specific pattern, although it takes a few more minutes, the outcome is much nicer.
Anyway, I pinned the entire strap just the way I wanted it to be, and then I made two stitches - one on each edge (in case you never done it before - you can sew on the pins, no need to remove them). First stitch for the hem, second stitch to flatten and shape the strap on the other side.

Step 3: Preparing Hem

I made a regular hem, again keeping in mind the fabric's pattern. I pinned the entire perimeter, then I sewed over the pins.

Step 4: Sewing the Side of the Bag

Now that the hem is done, I remeasured the fabric and remarked it. I cut the extra fabric and sewed on the mark. Then zigzag all the way...

Step 5: Connecting the Straps

First I've zigzagged the two edges of the strap, and then folded it inside.
I like using a strong stitch whenever connecting straps. I use this three-times stitch, making a square mark on the strap. After connecting the first side, I've measured it again, marked the exact location and sewed, again thinking about the fabric's pattern.
You can alway add a liner, if it doesn't feel strong enough.
You're done!