Just need to be clear about whether we're talking about renewal (ie old one already expired) or revalidation (within 3 months of expiring).
Renewal - need the proficiency check (LPC) with an examiner as per F3's note with the potential addition of some navigation if you have done no recent x-country.
Revalidation - as per Br/MikeCR but remember you must do at least 1 hr in one go with an instructor and you must get (a) his/her signature in your logbook and (b) an examiner to complete the revalidation form BEFORE your current validation runs out.
For revalidation, I think it's worth a discussion with the instructor on what you could most usefully practise (eg LPC items!?).