This Instructable is as much of a technique as a DIY project. The idea is straightforward; if you add weight to your tripod it will be more stable. By installing a hook from the center column of your tripod, you can hang whatever weight you happen to have with you, be it your camera bag, backpack or maybe even a water bottle or two. You don't necessarily need a hook-you could definitely improvise something else like tying a small rope around part of the tripod- but it is much more convenient so you'll end up using it more often.
Step 1: Disassemble Tripod and Get the Necessary Parts
First up, if possible, remove the center column from your tripod (you may need to also remove an end cap from the center column as well). This will make it easier to select the proper hardware and to drill the appropriate holes later. Speaking of hardware, you'll want to get a bolt and nut which is wide enough to go through the center column. An S hook which is about one inch wide and two inches long will closed and hung from the bolt on one end, so make sure it is small enough that one end (when closed shut) can fit inside of the center column.
Remove the end cap from the center column if necessary
Remove center column from tripod
Get an S hook which can fit in the center column when closed
Get a bolt which is wide enough to go through the center column
Step 2: Assemble Tripod Hook
First you will need to close one end of the S hook. I did this with a hammer, but it may be easier to do with a large vise. It will need to be closed enough that it will fit in the center column, but still open enough to allow the bolt to go through the center.
Next, drill a hole through center column 1/4 to1/2in from the bottom. Go ahead and replace the center column in the tripod. Put the closed end of the hook in the bottom of the column, thread the bolt through the hook end and tighten the nut on the opposite side to secure it. Don't tighten the nut too much however, as this may deform the column, making it difficult to remove it later.
If there was an end cap on the center column, it will no longer be needed as the bolt head and nut will prevent the column from accidentally falling out of the tripod.
Step 3: How to Use Your Newly Improved Tripod
This last step is easy- whenever you pull out your tripod for a shot, simply hang something heavy that you happen to have on the hook. I've been using a 5lb lifting weight when doing some night shoots in the backyard, but if your out on a shoot, a backpack or camera bag would probably work just as well. The extra weight will make your tripod much more stable, allowing you to take razor sharp images, even if you are doing long exposures!
One more tip for DSLR users out there- every time you take a shot, there are two big sources of vibration/shake. The first is you pressing the shutter button. By using a remote shutter or setting a short timer (I use 2 seconds), you can prevent the motion of pressing the shutter from jarring your shot. The second source is from the mirror flipping up. Each time you take a shot the mirror which usually redirects the light from the lens up into the viewfinder is flipped up out of the way to allow the light from the lens to hit the sensor. To prevent this source of vibration, there is often a setting on your DSLR called mirror lock-up (or on my Olympus, more cryptically, anti-shake). This setting can flip the mirror up a couple seconds or so before the camera starts acquiring a shot, allowing the vibration to subside before taking the shot.