what kinds of techniques would you recommend to a foreigner if you want him to finally correctly pronounce the "TH" in a word...
It depends on the foreigner in question. If they're Icelandic, then it's no problem because they already have that sound. Spanish, probably not a problem either. Otherwise...
1. Get them to speak with a lisp.
2. Get them to stick their tongue right out as far as it can go, with their top front teeth resting on it. Tell them that that's 10. Then get them to just make the sound of the letter S. Tell them that's a zero. Then suggest they aim for about 4.
3. Get them to invent as many new sounds as they possibly can. If it's a real simpleton you're teaching, you can get them to invent new alphabetical-style symbols for these sounds that they can colour in with crayons. Eventually, they will invent the TH sound for themselves.
4. You may have problems with TH being mistaken for F. In this case, try saying it whilst holding your bottom lip with your finger and thumb to show how it plays no role in producing this particular sound. If they still don't get it, but they don't mind you doing this, you can hold their bottom lip whilst they attempt the TH.
5. Further from the last point, you could actually take their tongue and manually place it where it is supposed to be.
6. Move to a country where English, Spanish, Welsh, Icelandic, Faeroese, Albanian... are not spoken.