If you tell your clients that for a 12-month period, you will visit their property twice a month and carry out (or arrange to have carried out) works to maintain their property you can tell them that it would be for a sum of (say) £6000per annum, payable in 12no. monthly installments of £ 500. (This will ensure that you always have something coming in each month). Parts and materials to be supplied at trade+ 5% (or similar).
Write up the contract, check for spelling and grammar ad leave any spaces for the figures blank. When you have verbally agreed terms with your client either add the figures/terms and then print it out, or write them in.
Get 2 copies done. You sign them, the person with authority to make the decision for your clients signs them (very important point; it stops them wriggling in dispute), date them and then you keep one (in a very safe place) giving one to your client.