I use this pump from MEC - never seems to fail, and the gauge works out nicely.

I tend to feel when the tyres get low, so I go by that - depends how heavy I've hit weird things (sometimes I notice hard pothole bangs squeeze air out) or how far/often I've ridden - I ride to work everyday but only usually need a refill every fortnight or so.

I inflate to around 100-110psi, I've had some weird blowouts with Conti Ultrasports (max. rec. is 100psi I think) so I try to err on the side of caution with that tyre, my front is apparently able to deal with 120 (Vredestein Fortezza SE) but 100/110 seems to work well for me(tm).

It's not rocket science, and you ought to feel when the bike is sagging as low pressure can give you speed/effort differences if they're getting fairly low or feeling more bumps than it should.