First step

The first thing to do is lay out your resin model and make sure it is all there. It sounds obvious but it is very easy to get carried away and start assembling the model without checking. A missing piece will then stop you from finishing it (which is far more annoying than finding that a piece is missing before you start). In some cases it will be necessary for the kit to be returned and this is much easier to do if the kit hasn't been started.

Preparation

The next step is to start preparing the model for assembly. This is a multi stage process, which I'll go through as different stages.

Washing

This isn't always necessary but I find it's generally better to be safe than sorry. Washing will help with the painting stage later. To get the best results from washing your model components I'd suggest using a mild abrasive cleaner diluted in water. I use a kitchen floor cleaner diluted in water. The models should then be given a scrub with an old toothbrush or something similar. A toothbrush is useful as the bristles are fine enough to fit into small gaps but firm enough to actually do something. I usually leave the components to soak for a little while before using the toothbrush. Then remove the models, rinse thoroughly and allow them to dry.
The reason for doing this is to make sure all traces of mould release fluids are removed. It also gives a very slight abrasion to the surface of the model that with some of the smoother models will help with paint adhesion.

Removing Excess Resin

Just like metal and plastic models there will be excess material that needs removing from the model before assembly. The difference being that with resin models the bits needing removal may be larger and must be removed with more care. These are the result of the casting process and unavoidable but all can be easily dealt with.

Mould Lines

Resin pieces are produced from rubber moulds that usually have a split line. Consequently there will often be a line on the model that shows where the mould joined. Sometimes these are fine enough that they can be ignored. To get the best results however they will need removing so that they don't show up after painting. To remove the lines use either a modelling knife, file or sandpaper and carefully scrape or file away the line. This shouldn't take too long and makes a big difference to the finished model.