Dating as an ISTP
So, you've been typed as an ISTP, congratulations! You're an enigma when it comes to intimate relationships. Until you understand yourself better, you will tend to come off as cold, uncaring and distant. Even if you have a deep love for someone, you will have to try hard to figure out how to show it. During younger dating years ISTPs tend to observe from the sidelines or place very little value in dating. Often they view it from a stance of “what's the point?” As a result, ISTPs tend to start dating late (averaging around 18-22 years old). ISTPs also tend to have large time spans where they have no interest in dating after a relationship has gone south. That is, until someone new comes along and drags them into another one.
ISTPs aren't usually the pursuer. Often they wait for someone else to make the first move. Which is one of the reasons they start so late. This is especially true for male ISTPs due to a social expectation that they initiate. If you want to initiate, do it. If not, don't bother. Forcing yourself to do something you have little interest in will usually end in wasted effort.
Learn about yourself
What the hell do you want out of a relationship? Do you simply want someone to have fun with and to relieve your "urges"? Do you want someone to be there waiting for you when you come back from your adventures? Do you want someone to keep you in check?
Ask yourself what you want. You're an ISTP, you don't really NEED people, so never get caught into the mentality that you NEED a relationship. That mentality will only set you up for disappointment or worse. Explore that which comes your way, but don't commit to it unless it's something that you're absolutely certain you want.
Learn about dating
Intimate relationships will be one of the hardest parts of an ISTP's life, minus the sex part... If you find yourself suddenly in a relationship, start doing some research. Ask questions of your partner, your friends, you parents, internet forums, etc. Figure out exactly what role you should be playing and how to do it well. You're an ISTP, this shit comes naturally once you know where you stand and have the tools to get the job done.
Learn about your partner
Well, if you care about them... Learn their MBTI type and love language. It's not just about what they like to eat, what they do for fun, how they like to be sexed and how screwed up their previous relationships were. Because ISTPs are so hard to understand, preemptively learning about how people tick is a good way to avoid relationship hell.
The first step is to realize whether they're an introvert or an extrovert, then lay ground rules accordingly. Especially if they're an extrovert. Extroverts will see you as a project or an anti-social puzzle made for them to solve. Tell them straight “I'm an introvert, I need time to myself sometimes. Go have fun without me, I'll be here when you get home.”
The second thing to figure out is whether your partner is a thinker or a feeler. Feelers will often want to know what you're feeling, accuse you of being angry when you're not, or make comments like “You hate people.” A thinker will operate much in the same way you do. You're spending time together, you're having sex and you're not yelling. Therefore, all must be well.
If you've determined your partner is a feeler, I hope you care about them. You have a long road of personal development ahead. You'll have to learn to be more vocal with your affections as well as your negative thoughts. If you can't express them, eventually a feeler becomes burned out trying to understand you.
What to watch out for
Beware of clingers. ISTPs don't do well with clingers, seriously. If you find yourself with a clinger, either learn them or leave them.
If you have no motivation, get out. If you don't really feel like seeing them for anything other than sex, end it (unless you can come to an agreement and make it completely sexual). Don't string them along. ISTPs are notorious for their love for all things sensory, and sex is probably the best sensor activity known to man. Be honest about your intentions.
Don't try to date just to “fit in.” If someone is harassing you to get a partner, tell them to shove it up their ass. You know what you want, and if you wanted a partner you would get one.
Complacency. This is the biggest cause of “ISTP rut” once in a relationship. Respect your partner, cherish your time with them and feel free to love them. But don't expect them to be around forever. The only person you can be 100% sure will never leave you is you. Take care of your interests.