The biggest thing is to get everything on the plan and in the specifications, if it's not there then it is not included, no good saying "But I thought that was included".
That means sitting down and thinking thru the whole of the construction of the house in your mind, get a pad and start writing then check the items are in the specs, every little thing, like toilet roll holders, towel rails, Power connection and if you plan on putting in air con later, make sure the power box is suitable for the expansion.
If you know anyone in the house building game in your home town, talk to them for ideas, even go to a display home, and ask to buy a full set of specs to one of their homes similar to what your building, might cost $50?? but it will give you a base to work off.
Remember if it's not on the plan or in the specs it will be an extra cost, or not done.