Whether you've done a restoration before or its your very first project - the following tips will save you lots of time and trouble, and keep you from making some very common mistakes.   
 
(1) Choosing your restoration project   
  Start with a vehicle that's easy to restore!
Choose a complete car with rebuildable or repairable parts. Replacement parts should be readily available. Avoid rare and unusual vehicles. Many antique and classic cars are really unique. The more unique, the harder it will be to find parts - if there are any to find!
This applies to customs, upgrades, and conversions as well. Most kits are only available for common autos, pickups, and unibodies.
Check with various parts locator services and car clubs to find out the parts availability for the vehicle you're considering. Restoration beginners sometimes select really difficult projects without knowing it, then become frustrated and quit.
Start with something easier (and cheaper) so you can spend more time working on your project, and less time looking for parts and info.   
  
 
(2) Before you get started   
  Don't start taking the car apart first thing!!!
Instead, take pictures of it's current state, make notes and draw diagrams before you take anything apart.
Write down data such as the location of small parts and exactly how they fit on the vehicle, brackets and their mounting angles, etc. Draw diagrams of the wiring, vacuum lines, and especially areas where photos don't tell the whole story. Note hardware changes needed (makeshift or temp bolts, etc.), and intended upgrades and modifications.
All this will help you immensely when you reach the final assembly stage, which could be years down the road from your starting date. Trying to remember, years later, exactly how a tiny part was angled or which clip goes where, can waste days and weeks of your time.
If the vehicle is extremely rare no one may be able to provide this information, and you'll be stuck hoping you got it right! Especially if you're taking the vehicle to a car show.
So document everything you can first - you'll be glad you did.  
 
  
(3) First Steps    
  Take out your list of parts that need to be replaced.
Aquire and replace the ones that will effect the way the body panels fit, including the body panels themselves, trim, grille, bumpers, etc.
Fit (install) all these replacement parts before the car is taken apart for any body and paint work.
Make sure the parts are adjusted to their final positions, the way they should fit when the restoration is all done. The replacement parts need to be fitted to the vehicle now, so that during final assembly you don't damage your new paintjob and the bodywork trying to get parts to fit.
If you plan to replace a section of the body you can fit the trim for that section when the section is replaced.
Fitting these parts in the beginning will ensure that everything goes back together correctly and easily at the end.
This is a really important step in the project, whether you plan to do it all yourself at home or have a professional do some of it.
You'll need to index most of these parts - when you reach final assembly you'll be able to install them exactly where you had them fitted.