A lot depends on your city.
* Churn credit cards. It's best if you can find a GC that lets you buy materials but, if not, it's still worth the minimums on several cards (apply for the cards after you apply for any loans, of course).
* Take pictures! When the wiring and plumbing go in the walls, take more pictures. Get your spouse or a friend to hold a tape measure in those pictures, too.
* The construction will take longer than planned due to the economy. Contractors are busy.
* Your city permitting office may be ass-backwards and a black hole of paperwork. Expect it and be pleasantly surprised if it's not.
* Get to know your architect's and/or engineer's drawings. Using a 2x10 joist instead of a 2x12 is obviously bad but a 2x12 joist instead of a 2x10 can throw everything off as well. Your GC or inspector may not notice so you'll need to pay attention.
* Shop the Habitat reStore for deals.
* Stain the slab concrete floor? It's inexpensive and durable. The concrete crew will need to know this beforehand so you can avoid extensive buffing.
* In the kitchen, put in blocking at the counter and cabinet levels. It makes installing cabinets far easier. Same with the bathroom.
* The little things add up. Make wise decisions about nice details and what sounds nice in the moment. It's easy to think, "Oh, it's only $250 more. That's nothing compared to my $100,000 budget." It's an ADU. It doesn't need granite countertops or stainless appliances.
* It's easier to run plumbing through a 2x6 wall. It's also better re:insulation.