It is really not a good idea to be getting any kind of payment at an exchange so you should probably cease this practice.
Mt Gox, Bitstamp, Bter are good examples. Please don't store in an exchange, you will loose your BTC anytime. There us no guarantee for that
One alternative would be to sign a PGP signed message, however I somewhat doubt you even have a PGP key, let alone know how to sign a PGP message nor even know what PGP is, or have one associated with your profile.
Donwload GnuPG and install it. It will be easy to create a PGP key with it. After you create, upload it to https://pgp.mit.edu and send the link via PM to 'DefaultTrust' saying that this is your PGP public key and if you're using a normal wallet, copy-paste your BTC address.
I would say a PGP message would probably be more secure then a bitcoin message as there are a number of reasons why a bitcoin private key could be transferred to another person, while there are virtually zero reasons why a PGP key would ever be transferred.
If a person is selling an account and if PGP is linked to the account, there is a chance the person will sell PGP and BTC address. There is many ways your PGP can be sold to get some $$$.
-MZ
