I always said if you do something do your best at it. If you find yourself not doing your best, then choose not to do it. If you can't choose not to do it, that means this is essential, and so go see #1 again. Do your best.

I think of it from the same perspective of business. Instead of doing OK support do your BEST support. If you are struggling, then choose to do no support - if your customers leave because support is critical, then guess what, you will probably get more benefit from doing your best.

So I think that advice works well as a way to really minimize your engagements, and figure out which ones are essential, by not just doing them just cause, by trying to cancel them entirely otherwise.