Best practice is to NOT leave equipment plugged in and switched off to give you a grounded surface because when you touch or hold on to this to ground yourself YOU HAVE NO CURRENT LIMITING SAFETY DEVICE in the circuit between yourself and ground, so if you do touch a live part (maybe the PSU is faulty and the outlet power switch has been wired in the non-hot line) - even perhaps something nearby like a faulty desk lamp or the printer that actually has the fault (and has blown up the PC), you do not experience a hand-to-hand fault current to ground, which is VERY DANGEROUS because the fault path is via your heart. This is why engineers who have to work on (high voltage) live equipment are supposed to keep one hand in a pocket - it stops them from getting a shock from the hand holding the screwdriver to the hand touching the equipment chassis; instead, they get a jolt down the leg which can flippin' hurt, but is less likely to kill them.
Proper use of the wrist strap (above left) is to either connect it to a grounding point (above right) to drain away any static you have generated OR to the metal chassis of the kit you are working on BUT the kit is still NOT grounded via a power cord and you are just using the strap for charge equalisation.
If you do not have the proper grounding wrist strap with safety resistor or an antistatic kit/workbench, just use the charge equalisation technique that Scott mentioned (with the equipment totally disconnected from the power outlet) OR just briefly touch a grounded metal pipe or connector and then work on the kit without holding onto anything grounded.
With regards to clothing, working naked is best but this may upset your friends/customers. Natural grown fibres/materials are next best - cotton or linen, but not wool or silk - so engineers that turn up in white cotton T-shirts and denim jeans with leather-soled shoes may look a tad casual but are less likely to be a walking static bomb. If you have to preserve a corporate appearance in front of your customers, a polyester suit may look the biz but may help you zap their server.