Don't dwell on the negative
From my experience, i find people that are positive most the time are more fun to hang out with. They see the good side of things, and don't dwell on the negative. When they describe positive things you can relate to, you associate positive feelings with that image, and you feel better about yourself. These types of people rarely/never make excuses in life, either.
On the contrary, some people I don't enjoy hanging out w/ a lot are ones who always focus on the negative often. They might talk about other people in a pessimistic way, they might complain about something, or make excuses for why they didn't succeed in some aspect of life.
1) As cliched tip, stay positive. Don't put other people down by insulting them. All it does it just make the person you are talking to associate negative feelings/images while you are in their presence. If you don't have anything nice to say, don't say anything at all. IT IS sometimes okay, however, to make a comment about something negative in a humurous way, example....
There is a classroom where the walls are just white, theres no windows, no paintings/anything to make it interesting.
A humurous way of interpreting the situation...."Man this room is really decorative isn't it?" (or something of that nature). This is a sarcastic/fun way of interpreting an otherwise negative situation.
A complaining way of interpreting the situation...."This room is so boring." Saying it this way just puts a negative feel to the whole conversation and puts a downer on a person who has to hear it.
In both situations, you are still essentially staying that 'the room is boring' but it's done in 2 entirely different ways.
2) Don't complain/make excuses. Don't you hate it when people always try to complain about stuff? whether it's their friends, their job....there's always something to whine about. It puts a damper on the whole situation and it doesn't really make you feel that great cuz you associate negative images while talking with them.
There is a big difference between complaining, and telling something which happens to be negative. Just the tone, and the way you say things like that have a big difference.
Let's say you did poorly on a math test.....a complainer/excuse maker might say...
"Man I did so bad on that math test cuz I didn't even get a chance to study. I had to go to soccer practice the night before, and then when i got home I had to drive my sister home.......etc etc...." A whole list of excuses/whining is really boring, and not exactly fun to listen to. Maybe these excues are true....but NO ONE CARES (except for extreme situations). You did bad on the test, that's it....no one cares why/how/what reason you did poorly on it. You don't have to justify yourself to anybody.
I think if you can avoid doing stuff such as saying negative things (whether they are negative or not) can really help your overall personality. If you MUST say something about a bad situation, then make a joke out of it. Try to make it as funny/optimistic as you can. Also, don't make excuses for yourself. The only person you are trying to justify the situation to is yourself, no one else cares. If you truly feel that the 'excuse' for not succeeding was legitament, you shouldn't feel the need to go around trying to rationalize why it went bad.