Make ‘no’ the default answer for new project/app review/etc. requests. New things should earn their way into the attention field.

Read a certain way, this could mean simply saying "No" to every new request. This may work well if you're running your own business, but it does carry the concurrent risk of cutting one's self off from potentially good opportunities. If the request demands an immediate answer, then "No" may be appropriate given one's current workload. However, getting the requester to document their request in detail — including what's expected of you and what's in it for you — and collecting that as a GTD "In basket" item for later consideration as to "Next Action" may be a better way of handling the situation.

For those who are employees, simply saying "No" isn't an option. However, there are appropriate ways to go about keeping one's plate at work from running over. First, if your plate is truly full and you can't take on an extra task or project request from your boss without duress, then you need to be able to prove it: documented project plans, due dates, etc. are the necessary evidence. Once you've shown your plate is full, you can ask your boss what they want to take off your plate so you can attend to the new task. If that's not an option, then you can offer meaningful suggestions as to other colleagues or team members who might be able to do the project. If this negotiation is handled respectfully, and both you and your boss are reasonable people, then you can usually avoid being overworked while simultaneously helping your boss get the new job accomplished and demonstrating your own management abilities.