I always score the tops of my plastic bases (like GW's or Shogun's) pretty severely with an exacto knife; this will result in better adherence of white glue, etc. (I use concrete patch, personally) If you're working with resin bases, you need to wash them with a toothbrush and dish soap before attaching your model to it (wash all resin models too, of course). You *could* do the same with plastic bases, but I find that scoring the base tops in a cross-hatch pattern works fine.

(on another forum I hang out on, the "Necro" smiley is customary when replying waaay after a thread has gone quiet)