Best light is up to an hour after the sun has set. USually too much light in the sky before then. Especially if the sun isn't behind some sort of cloud/pollution haze or very weak i.e during winter. If you're shooting the sunset before the sun dips below the horizon then I'd advise not shooting directly at the sun but use the warm, soft and very directional light to shoot around 90 degrees to the sun. You'll get some nice shadows to give depth to the image and the light is often lovely at this time (about an hour before sunset depending on where you are) If the conditionsual sunset. are right then 20 minutes or so before the sun actaully sets then think about shooting the actual sunset.
After the sun has set the cameras sensor will react differently to the available light than your eye will. If you can bear the boredom, cold and tedium of hanging around whilst the ca,era does a 5 minute exposure and then a 5 minute noise reduction, only to discover you've exposed wrongly and have to go back and do it again then multi-minute exposures will pick up the last vestiges of light and give you some wonderful colours in the sky that your eye may not see. Then it's time to turn towards where the sun set. You will need a tripod, remote release with lock for timed exposures. Some way of timing them (doesn't need to be too precise as a few seconds either way in a 5 minute exposure doesn't affect things) and probably some graduated ND filters to hold back the sky.
Throughout all of this you will probably need to meter on manual as once you get past 30 seconds your cameras meter wont be much help. One method I've found is to crank the iso up to 1600 or 3200 then double the exposure time for every ISO stop you drop back to 100. e.g 2" @ iso 3200 = 4" @1600 = 8" @ 800 = 16" @ 400 = 32" @ 200 = 1' 4" @ iso 100!
As the light dips your autofocus may start to hunt as well with not enough light to be able lock on to something. Time to start manual focusing. Have a google for hyperfocal focusing, read up and then use it. Generally, with a wide-ish angle lens then f8-f16 and set the focus distance to about a third of the way into the distance.
Not a definitive or exhaustive guide by any means but just methods of working that I use sometimes.
Hope it all helps!
P.s. get there a good 2 or so hours before the sunset is due. Last thing you want to do is miss the very narrow time band of great light by having to set stuff up and scout for a good spot!