I was told a story where a male westerner learnt Japanese from his girlfriend and ended up speaking more like a female. The storyteller thought this was hilarious.
What important differences should I look for between male and female speech?
There are almost too many to list, but the simple (grammatical) ones are:
Word selection is also important, but most dictionaries indicate this.
Probably too vast a question to cover exhaustively, but I can offer a couple pointers picked from experience (mine and foreigners I've met):
First-person pronouns (kinda obvious, really). Beside straight-up pronouns, the use of your own name to address yourself is a big tell-tale of female speech (some older grandpas can get away with it, but I doubt that applies to you).
Sentence endings: generally stay away from 「わ」 and 「かしら」. Conversely, there are some typical male sentence endings, such as 「ぜ」,「ぞ」, 「い」(as in 「だい」,「かい」)...
Some very connoted colloquial expressions (ちょー etc.), which are sometimes acceptable by teenage boys as well as girls, but will mostly make you sound like a teenage girl if you use them as a grown man.
Gender-infused grammar forms (especially for questions/rhetorical questions): is an incredibly complex and vast topic, with a fair bit of subjective thrown in, but I have often heard that the over-use of "soft" forms (such as 「〜のです」, 「〜です？」) or even the over-use of full verbal forms (when they can be omitted) tends to make you sound effeminate.
Overall, I'd say: pay attention to points 1~3 above and don't worry too much about 4 until you reach a fairly advanced level.
Edit: on a more general note, it is worth pointing that, if guy-foreigners using feminine speech is the most common side of this (by virtue of the male-to-female ratio being high in this category), the opposite (women learning from native men and using male speech patterns) also exists and is not any less derided. In fact, unfair as it is, Japanese (esp. of the opposite sex) will tend to be a lot more accepting of foreign-men-that-speak-like-women than the converse.