Follow up with clients once per month
Keeping an open dialog always helps. Make sure you talk general things than pure business. Remember this will help you understand your client needs and better understanding will help service them better.
Network with companies that compliment your niche
Find out what complements your niche. Learn some of the business/marketing techniques they use to build their business. You will be surprised to know that some ideas as relatively easy to implement. This will make you more proactive.
Build testimonials list for future clients to read
I am sure you will be doing these but sometimes we forget to highlight these. One thing to remember is to make these testimonials as real as possible. You will find that clients (no matter how happy they are) are a bit reluctant about this… you can offer them some free service in return for their testimonial.
Dare to be different; find what your niche is missing and fill in the gap
Ask around as what other services your clients would love from you. Asking the right questions will open doors for you. You might not be stocking such products but its worthwhile exploring supplying these by sourcing them from your friends in that industry. Make your customers happier.