There are two things you must do on a regular basis for analog tape decks (cassette or otherwise). These are cleaning and demagnetizing the heads and tape path. The second is far less important for cassette decks (multitrack or otherwise), and unless you move your deck around a lot or live near a place with a high magnetic anomaly in the earth, many people feel you can probably just forget about it (I'm not one of them, though).
Here's what you need, and since the Real Stuff isn't expensive at all (perhaps maybe it is in Chile), you should get good quality chemicals if possible. Don't worry too much about stocking up either, even a 2 oz. bottle should last years unless you're running a commercial studio or drinking the stuff (I personally recommended neither action).
I've standardized on TEAC/TASCAM chemicals perhaps because I've owned mostly TASCAM decks, but also because it's good quality stuff and readily available (Tech Spray brand may be easier to find on the net). I've found that TASCAM makes a nice set containing everything, model #TZ-261. Head cleaner and the fancy Q-tips are also generally available at Radio Shack, but rubber cleaner is a little harder to find, as is my Ultimate Tape Head Weapon. All you really need is the following:
How to Clean Your Heads
Do Not Use
Do not under any conditions use the handy-dandy "head cleaning cassettes" that foolish consumers have demanded ("Duh...I can't use a Q-tip...it's too complicated!"). They operate by running a piece of fabric over your precious tape heads, that, viewed on a microscopic level, is very much like sandpaper. Then you get to choose either the "dry" cleaning method (sandpaper a la carte) or the "wet" cleaning method (sandpaper a la mode).
What About Digital Tape?
DAT decks are entirely different, and for those I recommend you look up the DAT-heads site.
ADAT and DA-*8 decks are also entirely different, mainly because mechanically, they're really videotape recorders. So you should follow the manufacturers' recommendations with those, or anything else you think is better.
Here's a page which should be of great interest if you have any of the above digital decks.
The Ultimate Tape Head Weapon
I have had long debates with myself over the advisability of releasing this information to the public, but in the end...I won.
This substance was referred to obliquely in a book about recording I once found in a dusty shelf in the back of Electric Lady Studios, and at one time I actually found a bottle of it at a music store. I guiltily carried it off, knowing that I had found something akin to an original PAF pickup, and they didn't have a clue as to what it was really worth.
The bottle I acquired has a special "stealth" label printed in yellow on white so it literally cannot be read with the naked eye. However, using the blue filter on a pair of 3D glasses, I was able to read the inscription: "Teac Stainless Steel Polish SP-3".
It's a devilish combination of silicone and trichoro...no, I better not say the rest. But I will quote from the information on the back, which matches precisely what had been rumored in that book many years before:
To the true cognoscenti, nothing more is necessary to say. Don't expect
to find this magical liquid at the corner music store. You may have to
go to a crossroads and make a deal with a tall stranger at midnight. It's
been worth it, but the bottle is slowly getting empty...