If you are going back to your old job, check that your skills are up to date: they may have become rusty, or even been replaced by new technology and processes. You may need completely new skills. You should expect a little time to allow you to catch up, but life will be better for you and your employer if you arrive back at work as ready as you can be.
Get in touch well before your first day back to see if and how things have changed and what you can do to prepare. Try to arrange a trip into work in advance to find out what's new and who's who in the current team. Things are unlikely to have stayed the same. The relationships you had with certain colleagues before may no longer be appropriate – particularly if they have been promoted into a more senior role – and coming straight in and treating them as if nothing’s changed can cause complications.
If you are starting with a new employer, find out everything you can about what they do, how they do it and who does what to help you to fit into your new team quickly and smoothly.
If you need to make arrangements to cover childcare, illness or disability, make sure they are agreed and in place from day one. Even apparently simple things like planning your journey to work may present challenges you didn’t even consider before your circumstances changed. Taking care of the logistical details in advance means fewer things to worry about when you go back to work.