Like many many many girls, I have a soft spot for all kinds for make-up, beauty- and nail products, so a lot of trial- and error is involved within that area. Now I think I have optimized my nail routine, and the steps are:
Step 1: Remove previous polish with a nail polish remover (preferably a mild one, with as little of hazardous materials as possible).
Step 2: Polish the nails with a nail file, both the edges and the surface of the nail (if you also did this last week, then it's enough to polish just the edges and the "new" or bottom part of the nail).
Step 3: Use the nail polish remover again, wiping lightly over the surface of each nail. This removes both any dust that came from polishing the nails, and other oils that may reduce the durability of your polish.
Step 4: Use nail strengthener (I use Nail Envy Original from OPI which I buy on ebay here). Obviously this helps prevent your nails from breaking but it also helps prevent any taint of the polish to stick to your nails afterwards (as is likely to happen when you use strong colors).
Step 5: Paint the nails with your preferred polish.
Step 6: Use a top coat (I use the brilliant Seche Vite top coat which I buy on ebay here).
I promise you that the polish will last at least a week (given that you use a quality one from e.g. OPI, Essie or Chanel)! And this is exactly my plan tomorrow, when I try out my new Chanel color. Weird how a tiny little glass can just make your day :-)