Timing is everything. If you can say something before she's bought it, that's best.
Second best is telling at home while she can still change.
Never criticize her clothes and makeup if you're meeting her somewhere. It'll ruin the whole date. Then say nothing or if you have to mention her clothes, be diplomatically vague. (This goes for me too. I hate it when I meet my wife somewhere and she tells me she hates my clothes. Then I'm pissed and quite possibly self-conscious for the rest of the evening.)
Always criticize the clothes, the look, the color, the cut. Never criticize her for her choices and taste (or lack thereof).
If you're going to criticize her clothes, make an effort to complement her. The most well-meaning criticisms, if they're unrelentingly negative, really start to wear. Leaven the gripes with lots of praise and complements.
These rules generally apply for how you should deliver any kind of praise and criticism.