Actually these two letters form one sound in two different sounds, the more and the less guttural. The more guttural sound is used seldom in English, but there is for instance the famous Loch Ness and The sound for the ch in Loch is exactly what we're looking for.

Kachel tile   Bach   stream   sachte   soft

Well, and then there is the less guttural and there is nothing that it can be compared. Try this: take The sound of sh (like in shiver) and move the corners of your mouth outwards. You'll get a hissing sound and it should sound like the following examples.

ich I   Gicht   gout   sichten   to sight

Ok, the rule is:
after a, o, u, au, the combination of ch has the more guttural sound.
a fter äu, e, ei, eu, i, ie, ö the ch has the less guttural sound.

Actually the Swiss only use the first version of The sound ch. In Germany you can hear a difference between the two sounds, the Swiss don't use it. Let's have a look at these differences.

Both pronounce The sound quite the same.

Swiss version     German version   translation   Sache   Sache   thing   Rachen   Rachen   throat

Do you hear e the difference?

Swiss version      German version   translation   Ich   Ich   I