If you really don't remember, just be honest and ask them again.
The chance is that they are not likely to remember your name
either.
Just ask politely like this: "Hi, We met last week. What's your
name again? Sorry, I am not very good at remembering
names..."
After you found out the person's name, try to use his/her name a
few times when speaking to him/her. Eg the person is Tom, instead
of saying - "So where do you live?", you can say "So where do you
live, Tom?". Of course you don't use his name in every sentence but
try to include the name as often as you can (without getting
ridiculous) and hopefully by the end of the conversation you will
remember his name.
Good luck!! 
