Relationships are very complicated, make no mistake. Every person and situation is unique and the only way to figure things out is ultimately to throw yourself face first into the fray. However, when it comes to love relationships, people are generally looking for more. They want a relationship, something that means something, something that signifies that they are important to you. For a long time I've resisted that paradigm because I didn't want to sacrifice flexibility, but now I've begun to appreciate the value of having "a relationship" (or relationships) and what that means to people much more.
However, my idea of what "a relationship" is is still very different from what most people think of. Whenever I decide I want to have a serious relationship with someone, I commit myself to following some basic ethical rules-of-thumb that I've come up with from my own experience, as follows:
1. I won't give up on the person, even if they give up on themselves (perhaps especially). That means I won't leave someone for stupid petty reasons, and that I'll always do my best for them even if I don't necessarily know what to do. It doesn't, however, mean that I won't lose interest. I have had cases where I ultimately decided that we weren't compatible and ended a relationship on more or less neutral terms, but I do my best to avoid ending relationships on bad terms, and especially abandoning them.
2. I will not under any circumstance act out of revenge. This doesn't mean that I don't get into fights, or that they aren't shitty, but if something makes me angry I'll come out and say it up front rather than being a little shit and trying to do asshole things to 'get back at them' while pretending that nothing is wrong. That's just childish and solves nothing, and creates increasing resentment that will ultimately destroy the relationship.
3. I will never purposefully try to use a person's feelings as a weapon against them. That means I will never try to make a person that I relate to jealous, or to use their jealousy to get what I want if they happen to feel that way anyway. Jealousy is a pretty disgusting emotion, and can never form the basis of a healthy relationship. People who use jealousy to manipulate others are pretty disgusting, too, in my opinion.
The Next Level
A more difficult question is when to take a relationship "to the next level", which of course means sex. Normally my answer to this is "whenever it feels right", by which I mean when the relationship is otherwise sound emotionally, and when we are close enough that I feel like it (and it's typically mutual at that point). The first time with any person is usually very stressful, but after that it typically takes a lot of the stress out of a relationship. The girl I've been with the longest would say something like "sex is the most intimate act of connection between two people", although I'm not sure that I agree with that completely (I like post-sex cuddles just as much).
There are other cases where my usual answer does not suffice, however. Sometimes there's just too much sexual frustration, and it just needs to go somewhere, and sometimes a person might be too afraid to accept or believe in love right away. In those cases I might allow sex to come first, but it is always a judgement call (and a very difficult one at that). In those cases I try to take extra care to ensure that I can deal with the person's feelings effectively, so that eventually they can relax enough to feel love and enjoy everything more. "Eventually" is usually not very long, either, since my methods (when I'm not emotionally buggered myself) have proven very effective.
Types and Compatibility
Knowing what you want or don't want in a person is very important for making a relationship work. At one time I avoided making such a distinction because I didn't want to be a judgmental asshole, and to a point that's valid, but at the same time it's hard to be excited about someone who you don't really care for, and I'd prefer to be emotionally honest.
To me, personality is easily the most important factor. If you don't like a person's personality in general, if it doesn't attract you, and if you can't interact with them in a fun, casual way, then you're unlikely to get very far with them. Keep in mind that a person's emotional problems may obscure what they are really like, but you can usually catch at least glimpses if you pay close attention. There are also types that I like and get along with even though they aren't necessarily the most compatible, and some that I just can't bring myself to like. As long as you know where others stand and don't expect too much of them, things tend to work out well enough.
It is also important to know what you don't like. If someone has some trait that seriously turns you off, it will be hard to take them seriously emotionally, which usually does not lead anywhere good. For example, my 'main' type is probably intelligent, highly literate girls with an absurd sense of humor and preferably real interests. I also like shy, sweet types, although my compatibility varies. On the other hand, I absolutely do not like girls who are dumb, religious or excessively shallow. There are some exceptions, and it is more a spectrum than black-and-white, but if a girl has one of those traits it significantly hurts her chances of ever being very important to me.
The ugliest and most hazardous thing to look out for in any relationship is passive-aggression. This can be difficult to spot, but most commonly it shows up as things like fishing for compliments or attention, whining or unreasonable expectations. The worst thing about passive-aggression is that the person who is being passive-aggressive will almost always leave themselves some room for plausible deniability so that they can weasel their way out of taking any responsibility for their actions. They try to manipulate you into doing something, and if you do it then they shit on you and take you for granted, if you don't they may try to guilt trip or bribe you, and if you try to pin responsibility on them then they try to flip things on you. Passive-aggression is often, but not always, a huge pain in the ass to deal with. It takes delicacy and a firm hand to keep them from taking control of the situation and to discourage their manipulative behavior. There is also no guarantee that you will succeed either, because they can always find someone else who will enable their behavior, and they may even use this against you as an additional form of manipulation.