I just use permanant marker and make bold letters. The rug toffee is wearing hasnt need proofing for two years and the post code is still on. I have tested the new rugs with lipstick as someoen suggested on here, and its still on :O
I wrote clearly on both sides of rug, bottom corner, letters about 2-3 inches in size. Clear enough to see, plenty of tippex left and still visible ten years later!
I used masonary paint...it was in the tack room so I plastered them in it. I have my postcode in 10" letters on it.
The first ones that were stolen had marker pen but in the dark they didn't stand out and the theives still took them. I used a tippex pen on the ones that went last week so they obviously never saw that but the 10" painted ones weren't touched.