If you have a setup with Git Flow (http://nvie.com/posts/a-successful-git-branching-model/), or similar, you often want to push your local branch to a different remote branch. In such a case the generic push rule (mentioned by mozart27) will not work as it always pushes your local branch to the corresponding remote branch.
Example:
I have a local branch "feature123" which is ready for review (i.e. Gerrit). Our integration branch is "origin/develop". If I use the generic push rule then "feature123" will be pushed for review on "origin/feature123", but what I really want is to push it for review on "origin/develop".
Solution:
In SourceTree you can create a Custom Action:
Menu caption: Push for review
Script to run: cmd
Parameters: /c git push origin $SHA:refs/for/develop
Remember to add Git to your path to enable cmd to use it.
Then, instead of clicking "Push", you right-click your commit, and select "Custom Actions -> Push for review".
Ofcourse these 2 solutions can coexist, having the generic push rule ensuring that you always send for review when pushing to a remote branch, and the custom action for when you want to send a feature branch for review on the integration branch.