This is how I do it, took it from another thread(I wrote it, just copy and paste it)

Here's what I do. I'll take my glass pieces, pour a bit of salt in the dirty region, and some in a sandwich bag, I prefer ones with the sealing strip. I pour salt in the bag then some rubbing alcohol. I shake it for 5-10 minutes. I will blow in it see if any loose bits of resin come flying out. I then take some Q-tips with some rubbing alcohol on them and fish it around in the holes (carb, mouth piece, and bowl). The Q-tip will be covered in shitty black resin, I just discard it. After a few q-tips I'll shake it some more. Blow out chunks that are loose. Just keep repeating the steps till clean. Uh, you don't really need to refresh the alcohol or salt during this. Even if the water gets black, it'll still work. I did it with my glass pipe it wasn't cleaned for over 1 year+. I bought it from some guy who never cleaned it since the day he got it. There was so much nasty resin in that pipe. Lol ew. Well, have fun. I don't personally opt for boiling, not up for the risk of breaking glass with the fast temperature changes and such. Fuck it. . Here's a picture of my pipe and my bong, which I cleaned the day I took the pictures.