Reading this thread is a little depressing. All references to "league", as far as I can tell have been referring to "American" leagues, like APA... And advice is forthcoming about how to improve, using (American) league as the avenue, and people seem to be sympathetic.
And here I am in Germany, a member of the German league system, and it literally has none of the issues of most leagues in the U.S. If you suck... you play "Kreisliga", against other people in the same general skill level. Fail to field a team for a scheduled match? Your club gets fined like 500 Euro. For that single match. Massive peer pressure for people to commit to the team for the season, and to show up for scheduled matches.
And Germans take it SERIOUSLY. I rarely see an alcoholic beverage consumed before the match concludes. If your club is short on a higher level league team, and one is asked to "step up" to the next level to help out the club, the Germans still expect you to practice.
All the problems with leagues in the U.S. basically come back to the same thing: We see it as a "game", whereas the other countries see it as a sport. So, choices are basically, play for fun, and hang out with friends, or unless you are in an area with very very strong barbox players who maybe play BCA, your best option is to play tournaments. Full disclosure: I moved to Germany from Colorado state, which has very strong barbox players, and the vast majority of them play BCA. For me personally, I would probably only play BCA, if I were trying to improve. That being said, I had a 7 foot Diamond in CO, and simply never really practiced on it. Just not challenging enough.
It is our lack of taking the game seriously in the way the Germans do, that allows league setups like the APA to exist. Hokey handicap system designed purely to drive more teams, with no consideration of encouraging players to improve. There are no "tiers" to graduate to in American pool, unless you play BCA and go to nationals. The secret of improvement, as others have stated above, you improve by playing better players, and getting your head beat in. And taking that beating seriously enough to practice hard for.