All great suggestions so far. Personally I provide all projects with a year of free hosting. 80% of clients take this option which means a year after their project they will become annual hosting clients. Over the course of 5 yrs I have a retention rate of about 75% on hosting clients. (Some close shop, others move sites, etc…). This has turned into a very lucrative piece of my business. In the recent year I became a domain registrar and also provide SSL certs. This has allowed me to drive further business from this segment. I am currently signing all new clients with an SSL so this has doubled the profit from hosting alone.
I have also branched out and provide social media marketing help. I use hoot suite and help clients schedule postings out over quarter periods. This is one of the fastest growing areas of my business now and has expanded to FB ad management.
Another area I work in is print marketing materials. Business cards, banners, etc. I have been able to beat local print shops pricing and quality and have been driving all customers to get their print materials from me.
Getting repeat business from a former client is easier then obtaining new clients. It also helps create a relationship and that can lead to warm leads from past customers. Those are gold and should never been taken for granted.