What I'd do differently:
First, I would have listened to the architect, when she told me she didn't like project management, but then went on to ask her to project manage the thing! We did need someone to manage the project, even though the team of builders was very good - but there were things that she wasn't good at which caused some sticky issues, as she wasn't being assertive enough
Second, although you really do have to be honest about your budget, I would make it very clear that that is the budget and that there isn't any more money. Our architect wanted to use a specific kitchen maker which it should have been obvious to her we weren't going to be able to afford. We ended up using someone she had never used before, which also caused some issues. See my first point.
Third, although you have to be honest about your budget, you will have to accept that sometimes things will cost more, as things will happen that you can't have planned for. For example, when the back wall was being demolished for the extension, we discovered we had a dry rot problem, which obviously had to be fixed. So having a contingency fund will be important. (Rule of thumb, I was told, is roughly 10% of your overall budget 
)
Fourth, make sure that if your architect is choosing things for you, e.g. any specific fittings, or tiles, or colours, etc, that they have run things past you and you like them. We ended up with a new downstairs bathroom which, although I'm very pleased with, tiles and colours were chosen by her without consulting with us. (No idea how that happened, but it did!)
Fifth - actually probably the most important - is to ensure you have a contract with your builders/architect. We had one which outlined the overall plan, the timetable and also when payments were to be made. From memory, we had one payment up front and then the rest of the payments were scheduled throughout the build. Make sure you have an end date that you have agreed on - although you also have to accept that sometimes delays are unavoidable. However, make sure that the end date is realistic. We had a specific event that we wanted things finished for, but I think because of that, some of the finishing elements were rushed, and then not dealt with that well during snagging.
Sixth, ask your architect/project manager how often they are going to visit the site. For us, Alison lived just round the corner, so she popped in most days, which was very helpful. She was also able to keep a close eye on how things were developing, and to keep talking with the builders.
Seventh, choose your electrical items first and shop around for the best prices. Also, work out which ones you're prepared to compromise on. But also, don't scrimp. My attitude was that as we were only going to do this once, anything that was on my dream list of things, was going to be included! Some things may seem ridiculously expensive (e.g. my Quooker tap), but in the grand scheme of the whole thing, worth paying for.
Eighth, really pore over your designs/plans, and talk through them with the architect/project manager, so you're absolutely clear what's going where, and why some things have/haven't been included. For example, because of where the back wall was taken down for the extension, we lost a power socket, which means that in our living room, we only have power sockets down one side of the room. We also asked for an aerial socket in the kitchen, so we could put a tv in there, but didn't realise that to wire that in to the aerial, which is at the front of the building, we would have had to have the whole of the living room floor taken up. So we have a socket, but it's useless!
Finally, be brutal with the snagging list - don't pay your final bill until everything has been sorted to your satisfaction.
Hope that lengthy essay helps!