There are a few products I know I can rely on when I need to achieve a long-lasting, 'flawless' base.
First step, is obviously to prime my face. My favourite primer at the moment is the Smashbox Photo Finish Foundation Primer. I've come to discover that primers don't particularly make my foundation last any longer, but it does help achieve a smoother, more 'perfected' base to work on.
Then I like to apply concealer first. I know a lot of people say apply it after foundation, but doing this just tempts me to apply more foundation. My 'holy grail' of concealer and foundation combo is Make Up For Ever Full Cover Concealer and Nars Sheer Glow. Both featured in favourites posts here and here respectively. I've found the Real Techniques Expert Face Brush to be, err, expert, in applying higher coverage foundations, and their Setting Brush good for applying concealer under the eyes (and highlighting) although sometimes I use my fingers just for speed.
The final step, to ensure my makeup lasts, is to powder. My powder of choice is the Rimmel Clear Complexion Powder which I find to be just the right amount of mattifying, without being too heavy (which I find the Stay Matte version to be). I like to apply this in a pressing/stippling motion with a kabuki brush, as opposed to brushing it on - I find this prevents streak marks in my foundation.