I'll do this in a step-by-step guide and add some commentary here and there.
1. Install wine 1.3 or higher. This should be in the repositories under Linux.
For Macs, follow this guide:
http://www.davidbaumgold.com/tutorials/wine-mac/
Alternatively, install WineBottler. I believe it's outdated, but I guess it's capable.
2. Get a copy of GTA San Andreas and install it by running "setup.exe" with wine. Once you're done, replace the "gta_sa.exe" with a cracked one. It just won't work otherwise, even if your GTA:SA is legit.
You can get to your GTA folder by going to your home folder and pressing CTRL+H (or selecting "show hidden files") and navigating to ".wine/drive_c/Program Files (x86)/Rockstar Games/GTA San Andreas". The dot in front of ".wine" is important.
3. Download SAMP and install it as you would under Windows.
4. Run "winetricks". This may appear as a program, but you can run it from the terminal by simply typing "winetricks" without the quotes.
Notice to Mac users: I don't know if you have winetricks. Try and run it from the terminal first. If that doesn't work, please Google around a bit.
5. Select "select the default wineprefix"
6. Select "install a windows DLL or component"
7. Highlight "d3dx9" and press ok. It should take a while to install. When it's done, try to exit winetricks.
While you're at it, you can also install "corefonts" through winetricks. To do so, select "install a font" while in the default wineprefix selection. Highlight "corefonts" and press ok. This will slightly improve your fonts IG.
8. Start "configure wine". This should appear as a program/application in your desktop environment.
As for Macs, I once again don't know if you have this. If you don't, well, Google is your friend.
9. Confirm whether d3dx9 files can be found in the library. There should be a whole list of them.
10. Select "graphics" and highlight "automatically capture the mouse in full-screen windows".
11. Also highlight "emulate a virtual desktop" and insert your screen's resolution.
12. Select "audio" and fiddle around to see if your audio devices work.
13. Press "ok" and run "gta_sa.exe". It should start smoothly now. If it doesn't, post below.
14. Select your computer's resolution in the GTA session. If it's not available, simply restart GTA and it should appear.
15. Shut down GTA:SA and go to this website:
http://www.gtagarage.com/mods/show.php?id=8321
16. Download the files and put them in your GTA folder. It should ask whether you want to replace a file. You do want to, so press "yes" or "ok".
17. Rename "samp.dll" to "samp.asi" in the same folder.
18. Start SAMP. It should work. You may have to redo your resolution settings. Starting SAMP without ever having run GTA:SA first will not work optimally, so please don't skip any of the previous steps. Also, when starting a SAMP instance, you will have to alt+tab in and out of the game to enable keyboard input. This is a minor thing, really, but noteworthy.