What truck covers work best?

[sambudo]

I park my truck outside and would like to know what your using or doing to keep the rain water from pooling up in the bed area? Suggestions appreciated! Is their a pole system like a tent that keeps the cover tight? I'm currently using posts in the midle of the bed to deflect the water. I've tried weights etc.. It rain's a lot in the PNW! I like to keep it covered, if not covered after awhile it starts to grow moss in the crack areas. Let me know your ideas!