Gaffer tape is the best thing.
Go to a shop and try some of them before buying it. Some of them leave nasty residue on the camera...
You have to make ya camera look like it's an old crappy one.
So cover everything with gaffer and try to make the gaffer look pretty old. Try to rub and scratch it
with scissors or anything else.
Get a crappy and discreet strap, like the ones you can find on old film cameras. Or even worse...get a Pentax strap. That's funny but real. Thieves prefer a lot Canon/Nikon !
You could even try to stick some advertising stickers on it ? maybe.
Once you'll be in a "dangerous" area, act like you don't care bout the camera. That may be hard to do but it is important.
Here's mine :
Genuine old one...
To get an even more "crappy-old-one-look", you may take off the little cover thing on the left side and taped some gaffer instead.
I don't know where you're goin but if you go to some "real serious" place, you have to know that if some thugs want to steal ya camera, they'll get it regardless of weather it looks like...it is a big one, bigger than a compact, then it must be expensive. In that situation, just give it to them. That's sad but the best advice i can tell... You'd be happier once alive but without your 5D than seriously armed (or dead) without your camera.
Take care of you