If, for some reason, you need a quicker response than waiting for your weekly review, you can use OmniFocus to manage OmniFocus. Continuing with the example Gardener laid out, at the end of the Research brake lines project, you would add an action that tells you to make the Brakes project active again. You can even put a link in that action to the project to make it a bit easier to find.
To make a link to a project (or action), select the destination in the outline by clicking on its row handle (the dot at the left end) or the project icon, if a project. Now do Edit->Copy As Link, go to the place where you wish to insert the link, click, and do Edit->Paste. The link will look something like
omnifocus:///task/awtE5FoW3h_. Whenever you click on that link, OmniFocus will open a new window and take you to that target.
So, in our example, you finish up your brake line research, you encounter the action that tells you to take the brakes project off the shelf, and you make it active again. Might also leave yourself some instructions in this case to add any appropriate new actions to the project, based on the research.