This is what I do and it works well for me. I put it away for a while and I don't look at it at all. I work on new things - that is vital. Never stop. Don't let doubts about one piece of work poison your creative juices. Then, when I'm ready, I look at it objectively, as though someone else had written it. I revive it. Nothing is ever lost or wasted. I breathe new life into it.
To do this, I cut out bits, maybe removing a character that isn't doing his or her job. I alter the order of events perhaps or change the pace. Speeding up the action can help. I delete a scene or two that isn't helping the plot to progress. I check for long stagnant patches and improve their flow. Often, I add background history to flesh out a character who may appear a little thin.
It's like snipping ragged ends off long-neglected hair. Or tidying up a room after the school holidays. You find hidden crisp packets, ancient mugs, piles of watched DVDs out of their cases. You pick it all up, sort it all into the right places, clean it and air it ready for the new term. It's much the same with writing. And tidying can be satisfying, so don't despair. Once you have launched into the mess and begun detangling it, you will start to enjoy the process very much.