Who was the best captain you ever played under? Hopefully you learned a lot from them. If you didn't you might still be making these mistakes:

1. Ignoring advice. Advice is a funny thing. There is always plenty around when the team is doing well, but when you are getting soundly thrashed the captain is on his own. Despite this a good captain always takes on as much advice as he can (even if he has to seek it out). Everyone has an opinion and often it's different from your own so consider everything.
2. Listening to advice. The problem with advice is that most of it is plain wrong and as captain you are the one to carry the can if you lose. So while you should always consider everyone equally, you can be confident in the knowledge that you will throw out 95% of it immediately. You might make a mistake, but at least it will be your mistake.
3. Not going with a hunch. Hunches work. Malcolm Gladwell wrote a whole book proving it. Often your unfiltered ideas are the ones to go with because you don't have time to doubt yourself. Think of it as live brainstorming. It might cost you a boundary or two if you get it wrong but if it works you will be the hero.
4. Different batsmen, same field. All batsmen are different, even at International level. Club players all have an even more limited range of shots. Cut them off as soon as you can and getting attacking fielders in the right place once you know a players strengths and weaknesses. It's very rare that a field needs to stay exactly the same to both batsmen. Your fielders should know it and be alert to moving around every time the striker changes.
5. Not trying to take wickets. If I see another club captain put a side in and set a saving one ring field to his opening bowler I might scream. Your default position is to bowl the opposition out. The more you do that the more you win games so keep attacking as long as is sensible. The exception would be towards the end of the innings where you have little chance of bowling the opposition out and are trying to either defend against a declaration or turn a loss into a draw.
6. Following the ball. Every captain has been guilty of putting a fielder where the ball has just gone. This is a sensible idea early in a batsmans innings when you are cutting off his stronger shots (see above) but if he is well set and you are just moving fielders around in desperation then the batsman is setting the field not you. If you find yourself in that position take a moment to slow the game down. Re-analyse where the batsman is scoring set a couple of hunch positions and cross your fingers. Don't keep following.
7. Not giving everyone a role. Most people play club cricket for fun. Your job as skipper is to make it as much fun as you can. This means making sure everyone in the team knows what job they have to do. The star bowler probably already knows but make sure with a quick chat what you are planning. The last minute call up lad in his first senior match might be batting at 11 and fielding at mid on all game but he needs to know how important his job is. He might be required to hit the winning runs off the last ball!
8. Treating everyone the same. We might all dress in whites but what's going on in our heads is different. A good captain knows how to treat individual fragile egos and get the best out of them through motivation, bribery, shouting, smiling, cajoling, laughing, confiding, deceiving and a hundred other ways. It all depends what works best for the 10 others in the team.