If recognizable data patterns in the plaintext do somehow weaken your symmetric encryption, then your symmetric encryption is pure junk and should not be used at all. Do not make your data format inconvenient and complex just to cope with the (assumed) shortcomings of the encryption algorithm; instead, use an algorithm which does things properly.
It is difficult to say more about it without a complete and precise description of the algorithm you use. However, in all generality, we may say that if you probably don't know enough to do things properly (or at least to know whether you did things properly). That's not to be depreciative of your abilities: cryptography is a very specialized subject, and it has the unique characteristic of not being testable. Nobody can really know whether what they implemented is secure; they can test that it works but not that it cannot be attacked.
This applies to trained cryptographers as well. In fact, what distinguishes a trained cryptographer from an "ordinary" developer is not that the cryptographer can design and implement a secure protocol by itself; he cannot either. The real difference is that the cryptographer knows that he cannot make something sufficiently secure by himself.
The way out of this problem is to use an established protocol. For instance, if you want to do "symmetric encryption", then take an existing standard which has been investigated and analysed by many people (e.g. OpenPGP). And then, don't even implement it yourself; instead, use an existing library which has also been reviewed, so that most bugs have been extirpated (e.g. GnuPG).
