A quick courtesy call after the job to make sure they are happy, youll find a lot will ask you to quote on other items as well as say my friend, mum, auntie etc may be interseted, get there number and ring them, also good for your ego been told how good your job was over the last spit and polish guy they used.
Sugest when finishing the job that you will give them a ring in 6 months and see if they need further work, this is great as you can then plan your calls around your existing work,arrange to do 2 jobs close together, poss offer a slight incentive to get them to have it done while you are working in their area.
John