The following are links that may be helpful when looking for hidden interiors in GTA:SA
These were all put together while posting in the 'Hidden Interiors through the Gym' topic on GTAForums.com

Interiors For Beginners
Things you should know.

First Steps to Interiors
A step-by-step exercise to get you started.

Hidden Tattoos For beginners
How to get to the missing San Fierro Tattoo Parlor.

Found Interiors List
A numbered list of interiors. These numbers are what are usually used on any maps created on GTAForums.
This list gives information on weak walls and indicates which Heaven the interiors appear in.

Heaven Groups
This is the above list with the interiors grouped into 'Heavens'.

MAPS
Links to various maps that have been made while looking for Hidden Interiors.

F.A.Q.
Frequently Asked Questions.

Liberty City Links
Information about getting to Liberty City in GTA:SA

Pre SA Interiors
A document of my limited experience of hidden interiors in GTA3 and Vice City