When you commit (check in) your code to the source control system there are some things you should do to ensure quality and trackability.
1) If your source control system supports change sets, that is commiting a set of files as a bundle, make sure you just include one issue in that bunch. You shouldn't fix a bunch of things in all ends of the project and then commit them all in the same change set. The change set may well span over multiple projects tho, because one change set should include all the changes done to resolve that particular issue. A change set should also be compilable upon commit and not rely on the next one to be able to work.
2) Before you commit your change set you should diff every file against the repository version and see that you only commit things that were intended to commit. It's pretty easy to commit code you commented out, temporary variable names or debugging code if you don't review your own commits. If your commit is in central parts of the application or very large it is good to have another team member sit next to you when reviewing the changes.
3) When you've limited your change set to include only one issue and reviewed all the changes done, you should write a short description of the content in your change set. This is written as the "commit comment" and will be visible when you look at the log for your repository. Since your change set only should deal with one issue it is easy to write a brief description of what you've done. It's also good to include an issue id if you have an issue tracking system.
I guess many readers might think that this takes a lot of time - but think of all the time you save due to the higher quality instead! I've done this with all commits for many years now and it is very uncommon that I add bad code to the repository. It's not at all uncommon that I notice bad code while doing my personal code review upon commit tho!
My biggest problem when converting to this more professional approach was to limit my changes to only one issue. Yet today there are times when I can't commit only one change as I've fixed two (or more) issues in parallell without commiting the first one - and when they touch the same file it isn't possible to have only one change in the change set. If one of the fixes are small I usually solve this by reverting the changes for that issue temporarily while commiting the first change and then redo the changes for the next commit - but once in a while I need to write a commit comment with the dreaded word "... and ...".