What are some good general tips for developing good practice habits?

I've tried to learn to play guitar a few times, and keep giving up due to lack of motivation, instant gratification, and poor practicing habits in general. I've decided to give another go at it (and really do believe I'll do a better job this time), but given my history, I'm a bit worried that I'll give up again.

So, what methods have you guys used to increase your chances at sticking to it? Obviously it will all come down to motivation and willpower at the end of the day, but surely there are some pointers those that are beyond the beginners stage could share?

Edit:

Thanks a bunch for the responses so far, they're really helpful. So, from what I can tell, it looks like there are 2 general "rules" to follow:

1. Establish practice as a habit. Sound obvious, but I guess the key is consistency; make it so ingrained to practice that you do it automatically, and feel lazy when you don't. Just like brushing your teeth or any other "automatic" behavior.

2. Small, reasonable goals. Don't beat yourself up for not being Jemi Hendrix after a week. Again, obvious, but the main thing here is that it takes enough time to learn an instrument that it's easy to not realize you're making any progress at all. So force yourself into realizing that you are learning something by picking a small measurement.

Again, these things should almost go without asking, but it's helpful to know these are some common approaches/problems, and that we're not living in a vacuum. Thanks guys!