(My actual fixes for the detached feet are at the bottom but it wouldn't hurt to glance at the preliminary suggestions before tearing apart your trackpad.)
My first trackpad foot came loose after only 3 months of use. I did get Apple to replace the trackpad on warranty though I had to go up to Level 2 support since the "tech person" I got said that it wasn't covered since it could only happen from misuse (which I guess means placing the trackpad in one spot and never moving it from then on). [Thanks to my local Apple authorized repair shop who couldn't fix it on warranty because Apple wouldn't approve a fix or replacement by them. They told me that Level 1 wouldn't be able to do anything and I'd have to go to Level 2. Once I went up a level the tech person immediately arranged for a replacement to be sent.]
New trackpad, shorter warranty period, eventually same problem, now I'm on my own. This time I took the back off th trackpad to reset the foot I'd opened it once a while before to adjust it so it wouldn't click too easily and used a great guide that was very clear about being careful about the three plastic tabs that hold the lower side in place. This time, not remembering clearly how to open it, I searched and found a guide that wasn't as clear and I managed to break off all three of the tabs. (AFTER his step by step guide on removing the back of the trackpad the writer casually mentioned that perhaps it was best to begin opening the back at the top in order to avoid breaking the tabs. At the end, after the steps? We've had cut and paste available to edit what we write since the first Macintosh was created! BAD WRITER!) So I'll state here the two major lessons of trackpad (or any repair):
* (Trackpad specific) BEWARE THE THREE PLASTIC TABS AT THE BOTTOM! START FROM THE TOP AND CAREFULLY LIFT OFF THE BACK WHEN YOU GET TO THE BOTTOM!)
which leads to the next couple more general rules:
* FIND A GOOD GUIDE. Look at more than one guide to get the best informations you can!
* READ THE ENTIRE GUIDE BEFORE STARTING. Better yet, read more than one guide and print the best you find if necessary. Add notes from the other guides your read so you'll see them when you need to.
That may seem like overkill but I replaced a laptop screen using two guides, each of which mentioned something important that the other did not. I replaced the screen without anything going wrong. (And I can state from my own experience that soldering those little tiny microphone wires is not something you would like to do…and they and other things may be sturdy when in place yet very fragile when exposed during a repair.)
(one more Trackpad specific note (but a positive one):
* Ignoring the lovely detailing such as those buttons that look so nice until the third or fourth time one or both fall out, the Trackpad is a sturdy device overall. Several times during button replacement or adjustment I have slipped with a screwdriver or other tool and scraped or landed on the internal circuity in a way that felt like I must have finally done it in. Despite the missing tabs which caused the back to slip off now and then or the constantly roaming feet that required opening, closing, slipping and gluing things over and over the trackpad keeps functioning as it should. Don't stab at it to prove it to yourself but don't run off crying before you test it to see if all is OK. It really is a sturdy item (despite those stupid feet that must have been designed by the same person who created the totally round, totally frustrating mouse of an older (very round) iMac or who designed the new flat key keyboard and hid it inside such artfully beautiful packaging (Made by Apple in California!)
But after seemingly endless repairs to the trackpad's feet I have reached a solution where I can't remember having a problem for several years: I finally took the feel that came loose and placed them over the holes on the outside instead of inside. At first I carefully glued them (which didn't last) and then carefully put tape around the edges, leaving the operational middle of the feet uncovered to allow them to work correctly (which didn't last). Finally I just placed the feet in the right place on the outside of the trackpad case and laid a piece of semi-flexible electrical tape right over each foot, not tight enough to hold the foot in a depressed condition but not extremely loose either. Since then I've never had a problem with them detaching again. An added benefit for me was that clicking them required a slightly more force than before the fix and I no longer suffer from frequent accidental clicks when I'm trying to tap or drag.
I imagine that it would work just as well if a foot was lost and replaced using one of the suggestions of other commenters. I think it required one slight adjustment of the tape to get things to work as I liked. (I like having the click and tap options since I use BetterTouchTool and have made taps and clicks do different functions. If you only want to tap and dont' need to click then removing the feet as someone suggested above may be a good way to go.
Also, having lost the tabs that help hold on the back and tiring of having to re-glue it in place now and then I added a very narrow strip of transparent tape (and just the cheap transparent tape used for general taping in an office or home) around the back to hold the curved edge of the back to the very thin aluminum curve of the top of the body. (To do so, I placed the tape and pressed it down to conform with the trackpad letting it stick out from the sides and bottom and then used a razor blade or X-ACTO knife to trim it tightly along the very edge. (Both of these fixes work so well for me that I actually had to examine the trackpad to remember how I'd fixed it successfully!)
WATCH OUT FOR THOSE 3 PLASTIC TABS!!!