Tower Controller described a loss of separation event between a landing aircraft and a vehicle previously given runway clearance; the reporter noting established memory aids were not used as directed.

Narrative

I relieved the previous controller who included in the briefing that 2 trucks were on the field grooming and had clearance to enter the runway. With rapidly changing weather; the pilot of a Cherokee; who was VFR on a right downwind for Runway 24 called for landing clearance. My attention was drawn to the aircraft because of the weather and a bad transponder was causing the aircraft's tag drop from the D-Brite. I was having trouble verifying the aircraft's position; finally I was able to see the aircraft. I cleared the Cherokee to land on Runway 24. The aircraft landed and was at taxi speed approximately 1;500-2;000 FT down the runway. At that time; a truck asks if he was OK with the aircraft cleared to land. I advise yes and asked him to call the Tower. I immediately notified the FLM. Recommendation; the incident could have been prevented if our main memory aid would have been used. The Controller I relieved did not use it. It was my responsibility to make sure the position is set up the way I needed it to operate safely. Other factors were slow traffic; weather and multiple vehicles on the movement area. We often let our guard down when things get slow. We are taught to react quickly in this business; however when it is slow; it is good to slow things down. Those are times when you have to take a look around before you speak and scan scan scan!!! The 'VEHICLE' sign works great when used. I think that velcro place cards may help or a manual red flashing light on the panel to alarm you that vehicles are on the field.