Painting - Badly peeling, thick exterior trim

Our exterior trim is peeling down to the wood in many place and layer upon layer in other areas. To ensure a good repainting, can I just pressure wash the trim or should I strip and sand it. If s&s is appropriate, how thorough do I need to be. I want to do a good job that will last 10 years and not need re-stipping and priming again but I have a lot of trim to do and don't want to spend the rest of my life doing it! What's the most reasonable/good enough approach
here?

Scrape off any loose paint and rub down thoroughly with a coarse sand paper until smooth..Prime bare woodwork before undercoating and gloss. I use Dulux weathershield undercoat and gloss as this resists cracking and peeling in the future. 10 years is a little optimistic..6 years is more realistic depending on the weather conditions of where you live..If you try to use shortcuts with preperation then the paint can suffer in less two years and you will have to start again!!
Power orbital sanders and/or power drill attachments can be very efficient with removing loose paint.

Follow up question: (and thanks toptosher)
When I scrape off the loose paint, this leaves some paint that is still intact, but it's quite thick, so I have this "puzzle-piece" appearance. The remaining old paint comes off fairly readily with a hot gun so where I have begun, I talke the whole trim area down to bare wood, sand, clean, prime and paint 2 coats. However, on a big house this is a big job and I am uncertain whether all this effort is for simply aesthetic reasons or if I'll get actual pay-off in a longer lasting result. My concern has been that the unflaking paint is just waiting until I cover it with fresh paint to start chipping off.