If you only got a phalanx army without any supporting cavalry/infantry, or the support troops can't compete against the enemy's cavalry(because of significantly fewer numbers or quality), try "bending" the flanks of your line so that the flanks are also a line a spears. If you're defending, you'll have plenty of time to deploy your troops in this fashion, preferably on higher ground of course.

It doesn't work very well on the attack though, and I don't have a solution to that, always bring a significant cavalrysquadron when attack