Emery boards work great as tip tools
Duffman:
If you don't have access to a good-quality mushroom trimming tool (e.g. Joe Porper's
Little Shaver, or better yet,
Big Shaver), then a standard
emery board works just great:
At your neighborhood convenience/drug/pharmacy store, you can obtain these pretty inexpensively. You can even get the ones intended for nail finishing, where they have a very fine (600 grit) side for very fine buffing of a woman's nails.
What you'll do, is first -- take your time! Using the coarse side, at very nearly parallel to the ferrule (angled very, very slightly towards the tip and away from the ferrule), sand away the mushroomed portion of the tip. Take your time, and continually rotate the cue to ensure you're not "flattening" the spot you're sanding. In fact, you may at first just want to try holding the emery board still, and rotate the cue tip on it, again, taking care to keep the emery board at a very, very slight angle towards the tip and away from the ferrule. When you've removed the mushroomed portion of the tip and now have the sides of the tip as close as possible with the surface of the ferrule, you can flip the emery board over and use the fine side to buff it, using the same technique -- rotating the cue tip's side against it using the very, very slight angle you used when sanding it.
After you get a nice smooth surface on it, put the emery board aside, and get yourself a strip of brown paper bag (like your normal paper shopping/lunch bag). Wet the sides of the tip with a little water or saliva, and rub the sides of the tip briskly with the strip of brown paper bag. You'll notice this will "darken" the sides of the tip and buff it to a nice finish.
Voila!
Hope this helps,
-Sean