Switch back and forth between levels. If you are getting stuck or your score is getting even worse from game to game on one level, switch to another level and try that one, even if it's technically harder. You might even see some new obstacles that will help you when you go back to the old level.

In addition to the jumps and the falls and the spikes (the usual obstacles), also be on the lookout for collectible goods, as they will unlock new characters for you (one example of this being coins). Plus, watch out for unorthodox obstacles such as lava. You can sit in lava for about one second; any longer and you will end up dying.