How to obfuscate string constants?

We have an application which contains sensitive information and I'm trying my best to secure it. The sensitive information includes:

1. The main algorithm
2. The keys for an encryption/decryption algorithm

I've been looking at Obfuscating the code but it doesn't seem to help much as I can still decompile it. However, my biggest concern is that the keys used for encryption of serial numbers etc are clearly visible when you decompile the code, even when it's Obfuscated.

Can anyone suggest how I can secure these strings?

I realise one of the methods might be to remove any decryption from the application itself, while this may be possible in part, there are some features which have to use encryption/decryption - mainly to save a config file and to pass an 'authorisation' token to a DLL to perform a calculation.