Or invent your own encryption method or use compression so he can't figure out very easily how to reverse it
Making your own cipher is something that's very difficult. The strength of a cipher, much like the security of an OS, or the strength of an alloy, is not something that can be designed - it is something that must be proven over time.For instance the Feistel Cipher is usually considered one of the best. Not because it is the strongest or the most complex, but because it is the most thoroughly studied, and it is widely accepted that the best method of attack is bruteforce.