Try to take advantage of the current state of the cube. For example, in choosing an outer square for your first square, there are 24 different possible squares you could choose from. Pick one that is already partially (or fully) built. For the second square, it is usually best to build the adjacent inner square with matching colours. This will give you 9 possible squares to choose from for building the third square.
Of the 9 possible third squares, 3 match in colour, while 6 do not match. There is not much difference between choosing a matching or non-matching one, so again pick whichever square is closest to being fully built. If you pick one of the squares with matching colours, you will have 4 squares to choose from for the fourth square.