If you have a lot of circuits and accessories to install under the baseboard, consider mounting them on vertical drop boards at the rear of the layout. It's a lot easier than working upside down when you've mounted them directly to the underside of the baseboard, especially...
...if you unscrew them and take them somewhere more comfortable to work on.
Photo 7
In non DCC-Ready models, rather than use bare wire connections I always connect my decoders to a piece of stripboard (product code FL17T from Maplin) as seen here, stuck in place with a double-sided sticky pad. Although this isn't essential I find it makes life easier if you want to carry out any alterations later on - this unit has already had the decoder swapped for a sound-equipped one.
For trying out different decoders in non DCC-Ready models, a set of temporary connectors will make life that little bit easier. I use 4 short pieces of wire soldered to a couple of two pin sections of socket strip - product code DC17T from Maplin. These are then temporarily soldered to the selected loco - nothing fancy as you only need to connect the red, orange, black and grey wires. The photo shows a Zimo decoder plugged into these sockets and being identified as the best performer in a Hornby Class 86.