Remain consistant with how code is in the code base. For instance remain consistant with your whitespace, naming, commenting and whatever other rules have been established.
Find a fellow developer to help you review code together to correct potential mistakes and long-term issues that you may not have thought of
When writing code at the first pass if you feel that the piece you wrote could be written better, don't just ignore it, consider making a note to refactor.
It is beneficial to at least occasionally have pair programming sessions. When pair programming it is much easier for both developers to have focus on the task at hand, as they will have each other to keep them on track. In addition the quality of the code will improve as each developer can make notes on the new code as it is developed.
Unit tests are a big help in making sure that code quality is maintained. While unit tests won't catch all problems, and will likely not catch "stylistic" problems with your code, when a code base has well maintained unit tests, it helps you to make sure that you didn't unexpectedly break anything when implementing new code, or attempting to fix old code.
Implementing continuous integration is a good way of supplementing well maintained unit tests. At times developers may not remember to run their unit tests before pushing their code into the code base. Continuous integration software can help you ensure that everyones code passes your unit tests, whether or not the developer remembered it.