Hi,
Goose-down is amazing stuff - fantastic warmth, almost no weight, squashes down to nothing and all natural. However, as you suggested, it will loose performance if you allow sweat, body oils and general grime build up within the down. Keep it in good nick though and it will look after you and it lasts years.
I've cleaned my own jackets a few times, but it was a real pig.
Firstly, you have to use pure soap - either soap flakes or Nikwax down wash. Never let detergents come into contact with the jacket, this can ruin the down. As mentioned above if you are going to use your own washer then run it on a couple of empty cycles to remove detergent build up. Don't skimp on this bit - run them! After that just follow the instructions on the bottle.
Once washed, do not wring out the jacket. Just a low spin will do but this will also leave plenty of water in it. Do not wring it out, or have I mentioned that? Tumble dry on the lowest setting you have, make sure it does not get much above body temperature. Yes, this will 'age' the down if you get it too hot. Adding tennis balls helps beat the down up and stops clumping but this can take ages. And ages and ages and ages. The last jacket I washed It took 12 hours to dry!
If you have got through all this, your jacket will be back it its old self.
However, leaving it to the pros can save you a load of time, but can cost a load of cash too. Last time I called Franklins they quoted me £35 for a wash and re-proof, plus the cost of mailing the jacket up there in the first place. Turn around was quoted as two weeks, but this was quite a bust time apparently. On the other hand their service really is top notch. You can also get it done through www.mountaineeringdesigns.co.uk but I am unsure of their charges. I think PHD might offer a sercive too?
For the last few years or three I have trusted all my cleaning to my local dry-cleaners, 'Saffron' in Bury St Edmunds. I'm not sure if these are a chain or just a local store. They offer a clean and re-proof for ski gear and down jackets / salopettes for around £9.50 all in. It is a week to ten day turn around but they have done a great job on anything I have given them.
Hpe this helps,
James