How much gas should you keep in your car? Also Car problem?

[Jen]

I usually keep it at 1/8th of a tank, but would it be better to fill it up completly once in a while? Next, my car has poblems starting most of the time, it won't start until third try, even when I have a half tank of gas. The car just cranks and sputters and stalls, but won't start until a while later, sometimes 15 tries! The car is a Hyundai Elantra 2004 and I hold the key for about 10 seconds a try and pump it after the 5th try, but it makes no difference. Could not filling it up with gas cause this problem? Thanks...