Don’t be a fake giver (giving only because you’re expecting to get something in return). You’re setting yourself up for disappointment if you’re keeping score for a game which the other person doesn’t know you’re playing.
If you think to yourself, “I did a, b, and c for this person, so they should give me x, y, or z when I ask in the future,” you’re asking for disappointment. Hopefully that person help you out in whatever way they can because you’re a good friend and they’re a good friend, but expecting reciprocity (especially when you do something for them now, but want them to remember your actions far in the future) is going to lead to frustration from both people.
Be caring. Be nice. Find ways to help. But if you can’t do something, or if it stretches you too thin, don’t feel obligated to do so. It’s ok to say no – and you don’t have to give them a reason why either. If you want to explain to someone why you can’t do something for them, at least they’ll have a better understanding of why you can’t. But they’re not entitled to know (just like you’re not entitled to knowing why they might say no to you in the future).