Firstly, have you been to see any of the work that the builder has done? This was really important for us to see an extension that the builder had just completed and to speak to the homeowners about the process.
I'd agree with being on site - we lived through it so we didn't have much choice, but its the decisions about little things that haven't been thought about - which order do you want light sockets in for instance, how did we want the edging done on the tiling, how did we want the floor tiles laid etc etc. I agree that the builder is unlikely to telephone you - they prefer face to face dealings - and even if they do, without you being at the house to make sure you're both on the same page, its easily to get things confused. Some of those decisions need to be made quickly so yes to being decisive.
Sounds silly but if you are the one who has planned it all (my H was not really involved in it much) and you have the "vision" in your head, make sure its you that the builder speak to. My H would have said "whichever option is easiest / cheapest" to any options suggested by the builder!
As well as having the plans, when its things like radiators, door handles, sockets & switches etc, draw them on the wall. The drawings are really hard to interpret even if you've gone into that level of detail.
Have a contingency. It will go over - sometimes because things crop up on the way, sometimes because you see the space and want a better spec etc than you anticipated.
Don't be a pain in the neck - a good relationship with the builder is worth its weight in gold. Don't change your mind, be friendly, make tea & lunch for them (or at least offer) if you're there, don't sweat the small stuff, be clear with your expectations.
Yes to payment of the completion of various stages. Yes to a final payment only once snagging has been completed and you've got building regs sign off.
If you are buying things yourself, ask the builder if they have a trade account / discount (Screwfix / Selco etc) or they know of any local suppliers that they recommend. If you are buying things like bathroom fittings / flooring / anything that has to be delivered, especially as you're not going to be there, have a conversation about whether they're happy to sign for deliveries / take them in. I made lots of online purchases but always tried to schedule deliveries when I was there. The builders might be at the property but don't take them for granted.
And buy all the actual builders (as well as the head builder) a great big crate of beer when they've finished and done a great job
. Good luck!