In my experience most places I see people saying that thinking about the spell stops it working is on sites where others cast on you behalf. There's a reason for this and it's a selfish one, it prevents negative feedback, if you say "the spell didn't come true" they can claim its because your thinking about it. In my own experience I don't find it a problem.
If you cast a spell to attract love as an example and then forget about it and spend the next few months locked in your house then it's not going to come true, on the other hand if you meet someone you like afterwards and start talking to them the memory of the spell may actually prompt you into gathering the courage to ask them out on a date. It's a good idea not to become obsessed by it or you'll start seeing the spell "working" when it's merely coincidence and you also have your own life to go and live but I wouldn't worry too much about completely forgetting about it.