This is an interesting topic, and one that I've asked about before. For heavily contaminated boards I tend to use water (maybe a little detergent to start, then rinse) and then finish with isopropanol, scrubbing with a brush to loosen harder debris. Often I'll use an air line or canned air to blast the fluid from under and around components - this has the effect of carying away solids with the fliud and also helps the drying process.
As you've spotted though, it's a labour-intensive process and I've been on the lookout myself for more efficient (both quicker and more effective) methods.
I'm interested in ultrasonic cleaning, but have not tried it yet. I've got an ongoing project to build a vacuum over for drying after cleaning with water (this being the hardest part) - I'll be updating here with how that goes as (if) it progresses.
I'd never heard of these "water picks", but they sound like they could be very useful, so thanks
dknguyen for briging these to our attention.
None of these really adress
fastline's need to clean in-situ though... I'd imagine that if your contamination is dry then you should get pretty good results with compressed air. Maybe with a little help from a brush (preferably a cardon-fibre anti-static one).
Would also be interested in hearing others' experiences.