This is a question we get quite often due to the huge numbers of builders available.
1. You should find a builder you can easily communicate with, with all the correct qualifications and who brings enthusiasm to the job. (Find a Builder specialises in this).
2. You should make your plans before the build and commit to them. Constant changes will knock you off target for your budget. Additionally, alterations take up time and slow down overall progress on the build. This can all be avoided by discussing your plans before your build begins. (Find a Builder facilitates communication between builder and client).
3. Make sure that you are realistic with your budget, schedule and your expectations. Communication is the key to a successful partnership with your builder, so make sure that you start it early and keep in the loop!