Well well targeted is basically several steps you need to think about:
1. Are you going to do it yourself or pay a company?
2. Pick an area and stick to it. Affluent area or more working class?
3. Quantity of leaflets - normally 5,000. - this will vary on your area but most places 5,000 is a good starting point.
4. If you go for 5,000 pick either 1,250 homes approx or 2,500 homes. If it was me I'd pick 1,250. Then plan to deliver 5000 spread out across either fortnightly or monthly over the same houses until you run out.
5. Select a new area and repeat above.
It's trial and error but you should see a return on investment if done in above way depending on area and market saturation levels.