I find that the times when it's most important to say "yes" are when I most want to say "no". I've learned to ignore the urge to turn things down and often go along with them. Despite what others might say, I think this works quite well for me. I also believe that it is part of the reason why I've accomplished certain things and partially why I am where I'm at today.
So I want to recommend that everyone tries saying "yes" when they are really tempted to say "no". I will explain what I mean by this using an example of what it would be like for me. Quite often, when I feel depressed/anxious and I'm presented with a situation or opportunity, I'd like to just turn around and have nothing to do with it. Due to the way I'm thinking or feeling in those moments, it seems easiest to avoid everything and do absolutely nothing. But I know myself better than that - I don't want to do nothing, it just feels like that. So I go against the feelings and do what I know to be true about myself.
As I write this, I am suddenly thinking back to the movie Yes Man; therefor, I feel the need to say that this is absolutely NOT what I am suggesting anyone do...haha. Well, maybe do it a bit, but not to the same extent! Saying "yes" does open so many doors for us. That's why my advice is to really consider things before we turn them down.
Ask yourself why you feel strongly about saying "no" to something. Do you have good reason for saying "no"? Or is it mainly because it seems too difficult, slightly out of your comfort zone, or because you're being slightly lazy? Of course there is always valid reasons to say "no", but I find it important to learn the difference between a valid reason and an excuse. This skill is so beneficial.
Saying "yes" to certain opportunities has helped me to hold onto the person that I am, even during extremely difficult times. If I were to give in to the urge to avoid everything, what kind of life would that be? If I only did the things that had to be done, how boring would that be?
The best person for you to get to know is yourself. Aim high, challenge yourself, and you may surprise yourself.