I started with ONE merchant - they offered me the same percentage with no customer hassles - if I switched from our drop-ship relationship to their new affiliate program. I made a lot of money and had a problem-free affiliate existence for several years
It wasn't until after I decided to join every merchant at Linkshare did my days change fromm one affiliate-related problem to the next
So... in hindsight, I would...
1. Make websites that I enjoy. This way, when problems arise, at least you're dealing with something you like.
2. Do my homework and research and compare similar merchants so as to avoid the "dogs"
3. Always have a fallback plan - for when a merchant changes its terms, switches networks, etc.
4. Develop a relationship with a merchant PRIOR to applying to their program, so that you both know who you are and what you do before you become "partners"
5. I didn't know these boards existed until about 2 years ago. Communicating with merchants and other affiliates is imperative to success. So, find appropriate forums.
6. Go to summits and conferences so that you can meet people in person. No matter how good your online relationships are, meeting someone in person is irreplacable.
finally...
7. Go with your gut. Even if everyone else says "this merchant stinks", go with your gut. Try. Experiment. Learn. Improve.