ejs is exactly correct. All depends on what protocols you are using ADSL, VDSL, FTTC? PPPoA or PPPoE. Single-box modem-router combined or separate modem and router.
Tuning will only give you at most 3% more speed, or 0% depending on your setup. Let us know _all the details about kit, setting, services, protocols, line type_ and we will be able to advise.
Here is the rule in a nutshell:
About what works / doesn't work: If you have a separate modem, 1492 is the safe choice. Otherwise always use 1500 for maximum compatibility with the world at large. Beyond that all else is tuning/tweaking and we would have to advise you on that with a knowledge of all your details.
Earlier posters’ tips are valid or not dependent on your setup.
Tuning/tweaking: the possible gains from tweaking are not large (3% or 0% max) and we don't know how to do it without your details. ejs' excellent advice is protocol-specific, as he says, and 1478 is an optimum for efficiency with PPPoA _only_. But we don't know if you are using PPPoA or not.
(Further reading: The wikipedia article on PPPoE has a section on PPPoEoA over ADSL written by your truly, which goes through all the arithmetic in gruesome boring detail. It might be worth a read even if you are not using PPPoE. see other MTU thread)