It's good to type 'nn' not just at the end of words but every time you want a ん. That way you can be sure you won't get こんいちは when you wanted こんにちは.
In fact, apart from the vowels, all the kana characters can be entered with two keystrokes. You just specify the column (i.e. consonant) you want followed by the row (i.e. vowel) that you want. So the 't' column, for example, is ta, ti, tu, te and to rather than ta, chi, tsu, te and to. It feels a bit odd at first (for example, the two-key version of fujitsu is huzitu), but in the end you're trying to type kana, not romaji.
As far as I know, the two-keystroke version of romaji is what the Japanese use. One advantage is that if you're used to entering ちand つ by typing ti and tu, it's pretty obvious how to get ぢ and づ; it's not at all obvious if you're used to typing chi and tsu.