Every time you finish a job leave the client with some flyers and/or business cards. Ask them to pass them on to their friends. There's nothing better than recommendation by word of mouth between friends and neighbours, so some literature as well, with all your details on it will jog their memories when they really need you.
Also, on all your literature, make sure you include your full address and a landline telephone number, as well as your mobile. When I'm looking through adverts for a tradesman, I ignore all those that doesn't include the above information. For all I know they could be travellers, and be 200 miles away if needed to be contacted after a job was done.
At this time of the year, if you service central heating boilers and systems, then concentrate your current advertising on this.
Good luck.