Douglas Cole has put together an excellent post at Gaming Ballistic that looks at what grappling is and isn’t. Many old school games have basic rules in them for handling grappling, but these rules are often one of the first set of rules jettisoned and house-ruled by GMs. Douglas himself has already looked at Grappling for Dungeons & Dragons 5e in the past, and made his own house rules for it.
For later editions of D&D, where grappling had a number of feats, there are have lots of attempts made to understand the flow of grappling resulting in forum threads and flowcharts. Skip Williams has also put together a rather complete document covering grappling that can be downloaded in PDF that although written for d20 games covers a lot of ground that may be appropriate for older games. Brandes Stoddard has also put together another approach at grappling for his own game that is a little more abstracted and some may find interesting.