Here are my tips.
Treat it as a business. Treat the builders with respect but be professional (as you would in any other job). Yes I would leave tea, coffee, box of biscuits and I would be friendly but at all times be professional.
I shared my written list of requirements, and I have gone into extreme detail, if you didn't specify it then you have to expect to pay for it. Don't assume anything, or assume that it will be done as a 'favour'.
Remember that you will be paying 20% VAT.
Everything should be done in writing so there is no confusion about what you agreed.
Agee a payment plan so you everyone knows what is expected before the next payment.
Check the site daily so there if there is anything that is not going to plan it can be discussed immediately. I have had very little snagging issues as they were sorted out on a daily basis. Keep a notebook with a list of your budget and any issues that you come up against so that you don't forget anything.
Meet the foreman once a week to discuss progress and expectations.
Before the final bill is paid, make sure that you have all the building regulations (completion certs and for electrical work you have the Part P paperwork etc).
Take photos before and after and also keep a 'mood board', to remind you of what you are hoping to achieve and it will help to keep some perspective (you might have days when everything seems overwhelming).
If you have a budget consider what will happen if you have to go over it. Even with my control freak planning I still went over budget for items that I have no control over.
Also if it might impact your neighbours go and speak to them first and explain what is going on. Listen to any concerns that they have.
Good luck.