Always use a face primer under your foundation. Also, find a primer that works best for your skin type and addresses any skin concerns that you might have. There are many different types for oily and dry skin as well as pore minimizing, blemish treatment, reducing redness, smoothing skin, etc.
Here are a few of my favorites (affiliate links):
Bare Minerals Blemish Remedy – discontinued, so they are on sale. The new formula is not as good.