So how do you do it when you wash tallis koton / tzitzis to make sure the knots don't unravel or do you retie it all after every wash or its not machine washable at all? If there's a better way to wash it what is the best way?

Wash it in one of those small mesh bags. 



also, wool becomes stretchy when it’s wet and shrinks as it dries, so when you get a new pair, wet the knots and pull them really tight. They will never get loose afterwards. 

Wrap the strings with silver foil (thats what my older brother used to do, dunno how successful he was...), or buy the Magen tzitzit

hand wash, baby, the only way to go!

Wow thanks so much for all these ideas. Will definitely have to try some of them out.

My קבלה from my Mama is azoi: You put it in the sink, put some dish detergent (not hand soap, that stuff is shvach) in, ya shmush it around the בגד- getting it all around, rinse it around, wring it and hand out overnight. 
If you wanna do it in the machine, put the strings in a sock. 
After either method of washing, while the strings are still wet, tighten ‘‘em. (Like Gevura said). 
לחיים!

Oh, and also, if anybody needs a new pair, you know who your dude is! (I can ship, if necessary)

Hi and welcome!

Some great ideas here, but personally I would focus on fixing them up first before trying to clean off the existing dirt.

Either way, kol hakavod for trying to salvage them and not just giving up and throwing them out.

Now that we’re on the topic, here’s a few tips I’ve found to be very helpful. 
Especially after trimming the strings (which are most easily done with ceramic scissors, by the way), coat the ends with either super glue or clear nail polish (the instant dry ones are great), but don’t put them in the dryer- this will cause ‘‘em to puff up. You can also coat them when they’re brand new, to avoid any fraying. 
You can do the same with the last knots to keep ‘em secure.