Buying a foundation is a nightmare for me, I not only struggle to find the right colour but also to find one that lasts. Many foundations claim to last up to 8 hours (up to being the important word) as often I find mine have almost gone within a few hours. With the average foundation costing around £10 it is not a cheap product to be able to experiment with.
The most expensive foundations do seem to last longer and many places like Debenhams will have counters set up to allow you to sample them properly before you buy. When thinking about this, I came to the conclusion that I was better off doing that than buying three or four cheaper ones trying to get the correct colour and wear length.
Adding a thin layer of powder to your foundation can make it last longer too, but adding too much can make the skin look aged and show up small wrinkles.