Cheapest way to make a tripod sturdy enough for long exposures?

[Socket7]

Forgive me, I'm very new to this astrophotography, I've been doing this for all of a single night.

I was looking out my window last night, watching planes come in to land at a nearby airport. I thought to myself. I could go out to the hills away from the light pollution and get some neat long exposure shots of "UFO's"

I took my tripod and my 70-200L, a remote trigger and wandered off into the night to take some long exposures. This is what I ended up with.
http://farm4.static.flickr.com/3628/3659594707_02349b689b_b.jpg

Cool effect, but not exactly the shot I was looking for! As you can see, I had some serious issues with camera shake (this is the best one, the others were far worse). After browsing around here today, I found a couple tools to I could use to dial in my focus in better, and that I should be using the mirror lockup as well (derh). I'm pretty sure that my tripod still Isn't going to be stable enough, even with the legs pulled all the way in.

What else can I do to stabilize my tripod without spending a lot of money? I know the right thing to do is buy a big heavy telescope tripod, but I'd rather save the money for another lens. Would suspending weight underneath the tripod help? putting sandbags over the legs? Forgo the tripod entirely and rest the camera on sandbags on the roof of my car?

I'm just looking for ideas under 40 bucks that will help me pull of shots like these. Any help is appreciated.

[Catanonia]

I assume you are using a remote shutter release with a timer.

Basically set the timer to 10seconds, then on the remote shutter cable, start the frame.

Mirror lockup happens and then 10seconds later the picture is taken. The 10 seconds helps for the vibrations to go away before imaging.

If you are not doing this, go and buy a remote release for 20 bucks and save yourself lots of time.

Also ensure there is no creep in the tripod. Mine crept with a big lense on board even though it was a tripod lense.

[A.S.I.G.N. Observatory]

On a light and flimsy tripod, your shake will be much worse if it is all extended to full height. If you don't need the height, keep it as short as possible.

I bang a tent peg into the ground or use a weight, then stretch one of those elastic camping straps with the hooks at either end between the hook on the bottom of the tripod and the tent peg or weight.

This pulls the tripod down nice and firm which greatly reduces any shake from wind and stuff.

Baz.

[Bill Boehme]

The center post on my tripod has a hook to suspend a weight, but I use a bungee cord to an aircraft tie-down anchor screwed into the ground. A lightweight tripod is going to vibrate to some extent no matter what you do because the camera and lens is a large pendulous mass suspended on the end of a flimsy cantilever (the tripod). Sandbags and weight or anchor will help, but not be a total fix.