Better prepare for a career in xyz. It's not the most elegant, but it's also not wrong.
Better is being used as an adverb to modify the subjunctive verb prepare. The subjunctive mood is used to express anything hypothetical or anticipated: wish, possibility, opinion. It often appears in company slogans: they suggest x will happen if you use their product or service. Another example of a slogan that uses a subjunctive verb with the adverb better is WalMart's: Save Money, Live Better.
Often the subjunctive mood is confused with the imperative mood (i.e. a command), possibly because they share a terseness, or sometimes a lack of subject, and both express something that has yet to happen. It's likely that you feel the example sentence is "wrong" because you are simply reading it in the imperative mood. Adding a few more words to our example may help demonstrate the difference:
- Subjunctive: (Follow these tips to) better prepare for a career in xyz.
- Imperative: (You should) better prepare for a career in xyz (or else).
My opinion is that confusing-yet-grammatically-sound sentences represent failure to communicate an idea and should be avoided.