A good addition to your toolbox is silicone grease. This lubricant is the preferred lube for most O-rings in the dental office. Syringes, vacuum valves, various internal valves of the delivery system or unit foot control, all will do well to be lubricated regularly with silicone grease.
Most O-rings used in the dental delivery system are made of a Buna-N or derivatives thereof. This compound responds well to silicone grease but can be broken down by Vaseline or similar petroleum-based compounds so they should be avoided. WD-40 is also to be avoided as it will surely break down O-rings. It is good as a solvent for corroded parts but should not be used routinely.
Given how they are used, the O-rings on your vacuum valves would do well to be lubricated once a week. The valve will need to be disassembled to access the O-rings and properly lubricate them.
The diagram below shows common lever-style saliva ejector (SE) and high volume evacuator (HVE) valves. Note that they each have 2 O-rings on the valve cylinder (the part that rotates to turn the valve "off" and "on"). There are also other O-rings in the base and (for the HVE) at the tip. All of these should be lubricated regularly, especially those on the cylinder. To access the cylinder O-rings, pull the lever off and push the cylinder out.
As you may be aware, syringe buttons also have O-rings. The life of your syringe and buttons can be extended with regular lubrication too. Depending on water quality, syringe buttons should be lubricated anywhere from quarterly to annually. To lubricate them, remove the buttons and smear a little grease on the O-rings.
Most other things should be lubricated "as necessary" i.e. when you notice sticking or other problems.
When lubricating, generally just a dab of grease on the end of a swab is enough. Use the swab to smear a layer on the o'rings to keep them supple and flexible.
It only takes a few minutes and a small dab of grease, but the life of your equipment can be dramatically extended with this simple process.