For tender engines, my preference is the so-called "American" system where the engine and tender have opposite polarity. This is achieved by insulating the wheels on one side only, and on opposite sides for engine and tender. One terminal of the motor is then connected to the engine, and the other to the tender. Providing there is no other electrical connection between the engine and tender, that is it.
The big advantage is that it does away with rubbing pickups altogether. The disadvantage is that the engine and tender bodies are live to one or the other rail, and there is the potential for short circuits if you double-head your trains, or buffer up your locomotives on shed. Insulating the buffers and couplings is a way around that problem, but it does introduce an extra level of complexity. I have also heard it said that problems arise if the loco is stopped straddling a section gap. The engine is then in one section and the tender in another, and potentially there is a current path through the engine and tender between the two sections that might cause some control problems. All in all, it depends how you plan to operate your locos.
The electrical connection from the tender to the motor in the engine can be by a separate lead (you can pretend it is a water pipe!), but it needs a plug and socket if you are to be able to split the two. Alternatively, insulate the drawbar at the engine and use the drawbar to convey the current. The drawing shows how this can be done.
The drawbar pin at the tender end is plain. At the engine end, the pin is a screw (about 8BA is right) secured to a frame spacer but insulated from it by means of a bush and a fibre washer. The drawbar itself is held in place using a light spring. This is probably overkill, but it is necessary to ensure that there is electrical contact between the drawbar and the pin at all times. The spring also prevents the nut coming undone.
If the weight of the tender bears on the engine (very useful in engines like 4-4-0s to improve adhesion), the weight tends to push the drawbar downwards, and it may be helpful to have an additional spacer like the one shown as a dashed circle to take the weight of the tender. Beware though, the spacer also needs to be insulated.
The other main option is the plunger pickup. There are commercial ones available, but I prefer the design of Tony Reynalds (Model Railway Journal No. 74), which is a bit more complicated but allows the pickup to be dismantled and serviced without removing the wheels. This is my interpretation of Tony's idea.
The insulating bush (Tony recommends Nylon but I've used Tufnol) is threaded and held in place with a retaining cap in brass or nickel silver. Tony used a 5BA thread that allows a generous clearance for a 1.6mm hole through the centre where the plunger goes. Another option is M3.0, which is actually slightly smaller. The spring should be light enough not to give rise to excessive friction, but strong enough to keep the plunger in place at all times. Polishing the backs of the wheels is definitely recommended when using plungers.