MASK

It is really important to prepare well your mask before the dive so that it won't get foggy. First you need it to be very clean.
Fresh from the manufacturing line, a new mask often has a residual layer or silicone covering the tempered glass which will create lots of fog during a dive. There are many methods to treat a mask including soaking it overnight in cola, using toothpaste on the inside and burning it with a lighter.
In my opinion the best way to remove that pesky extra silicone is to first burn it so that the lenses turn black. Be careful to stay away from the edges of the lenses with the flame as you can damage the seal between the glass and the frame. Rinse off the soot and then thoroughly massage toothpaste onto the inside of the lens and let it dry all night. The day after wash it with dish soap and rinse. You might need to repeat the process few times before it is efficient. Patience !
Also when you soap up your mask before your dive, be sure not to rinse all the soap out. You don't want it to burn your eyes but that little soapy film will prevent fogging quite efficiently.

REGULATOR

Before doing anything, read the manual for specific procedure. When you take your regulator of from the tank, clean and dry the first stage tank connection to make sure there is no dust or water on it. Blow dry the dust cap and screw it firmly back in place. Rinse the entire regulator in fresh water as soon as possible after use. Be careful not to press the purge button of the 2nd stage, which might open the valve and make some water seep into the 1st stage. The best is to dunk the second stages into fresh water, paying special attention the the mothpice and any connections. Remove the hose protectors from their place to rinse the length of hose hidden by this rubber sleeve.
Dry it away from the sun without twisting hoses.
Have your regulator professionally serviced in an authorized dive center every 2 years, or according to the manufacturer's directions.
Be sure to always have this technical maintenance carried out by a trained licensed professional. This piece of equipment keeps you alive underwater, so please be responsible with making sure it is up the the precious task.

BCD

Be sure to check your BCD before every dive. Are the releases well clipped? Is your cylinder band secure and safety strap in place? Does you LPI function correctly with both power and manual inflators? Does the LPI's direct system deflator button work well? Do the emergency dump valves quickly and effortlessly deflate the BCD?
After the dive give your BCD a bath on the inside and out as prolonged exposure to salt water can degrade the inner bladder as well as the seems and stitches. Rinse the outside with fresh water then partially inflate and dump out any extra seawater that may have gotten trapped inside. Be sure to use your emergency dump valves to drain the water as salt water can damage the direct system on the LPI very quickly. Now pour some fresh water into the BCD through the manual inflate mouthpiece to rinse the inside. have a little dance with your BCD as you turn is upside down and in all directions to move the fresh water to every corner of the bladder. Now hold it as high as you can with your emergency dump valve at the lowest point of the jacket and purge the water through the dump valve. Partially inflate and let it dry away from direct sunlight in an open air place. Store it in a dry room.

EXPOSURE SUIT

After every use, rinse your exposure suit inside and out in freshwater. If you have access to warm water, this will help dilute salt and chlorine more readily and flush residues out of the fabric.
Lubricate the zippers with Vaseline or silicone lubricant. Make sure to coat the length of the zipper by doing up and undoing the zipper several times after greasing it up. This can also help remove any small particles that can jam it up and damage it.
Dry and store it on proper hangers, large enough to prevent lasting folds in your suit. I always prefer to fold my wetsuit in half over the hanger instead of placing the hanger's arm's in the shoulders like you would with a coat. When the suit is wet it is quite heavy so hanging it the way you would hang up a shirt can actually stretch out the shoulders over time, deforming the upper half of the garment.
You can even get hangers specific for diving equipment with little hooks and clips designed for hoods, gloves and other accessories.

KNIFE

Even your blade is stainless steel, you need to rinse it properly after diving. Take it out of its protective sheath and if you can, open it completely. This will allow the blade to dry and prevent rusting and also can free small salt particles from the joints.
Dry it properly with a towel and store it in it’s case again so you don't risk damaging your gear or yourself.

FLASH LIGHTS

Rinse it with fresh water as soon as you can, so salt water doesn’t have time to dry on it and leave marks. Press the ON/OFF button few times underwater to remove any dust or salt particles and make sure the button doesn't jam up in either position.
Remove it from the water and dry it on the outside. Make sure your hands aren't wet and to open it gently so any lingering water doesn't get inside. Remove, dry and clean all the joints and lubricate the o-rings and threads. Put it back together and store the battery and the torch separately in a dry area away from heat.