Many people have difficulty with preventing the paint from their models chipping off. The solution is a couple of easy steps. These will not completly remove the possibilty of chipping, but they will greatly decrease the chance of it happening.
1. Wash your minis.
Mini's, both plasic and metal, have casting release residue on them. This can cause paint to come off. Make sure that you always wash your minis with soap and warm water. Plastic is particularly easy to do, as you can just wash the whole sprue.
2. Prime your minis. Priming ensures a bond to the metal of a mini, and gives you a secure surface to work on. Spray-On primers tend to work much better than brush-on primers. Be sure when you are purchusing your primer that it actually says PRIMER. If it doesen't... Well than its not a primer. The best brand of Primer I have found is "Dupli-Color Sandable Primer" and can be found at auto-parts stores in the US.(Sorry Brits, no clue what you folks have over there). Dupli-color works much better than Krylon or GW primer, and is still fairly cheap at around $4 a can.
Be sure to let your minis sit for a day before painting them after priming. Its good to let the primer set up and cure. This goes for any primer.
3. Seal your minis. Sealing is the last important step in ensuring a well-protected mini. While glossy coats actually protect the best, they also ruin the paintjob on a mini. Matte varnish is the best varnish to use in mini-painting. The best Matte varnish out there is "Testors Dullcote" and can be found at any hobby store, and most craft stores. Some people like to give 1 coat of Satin, than 2 coats of Dullcote to eliminate the shine while providing good protection. I personally just do about 2-3 coats of Dullcote and my minis usually don't chip.
If you follow these 3 easy steps, your minis should last you a good amount of abuse before the paint chips. Just don't throw them at your opponent.