Keep the airplane simple. Extensive avionics, besides being expensive and heavy, take a lot of time to install. Even small changes to the airframe can consume mind-boggling amounts of time. Just a fancy paint job can add several weeks of work. If you can spend a couple of hours every evening, your airplane will take less time to complete than if you work one long day every weekend.
Momentum is important.
Make your shop comfortable year round. Get your friends and family involved, at least to a point. An extra set of hands can save a lot of time, even if they are just shoving clecoes in holes or handing you tools. Make as many decisions ahead of time as you can. Especially when installing systems, you'll go much faster if you've thought it through and bought the parts before you actually need them.