When choosing words for your app:
Keep it brief.
Be concise, simple and precise. Start with a 30 character limit (including spaces), and don't use more unless absolutely necessary.
Keep it simple.
Pretend you're speaking to someone who's smart and competent, but doesn't know technical jargon and may not speak English very well. Use short words, active verbs, and common nouns.
Use contractions. Talk directly to the reader using second person ("you"). If your text doesn't read the way you'd say it in casual conversation, it's probably not the way you should write it. Don't be abrupt or annoying and make the user feel safe, happy and energized.
Put the most important thing first.
The first two words (around 11 characters, including spaces) should include at least a taste of the most important information in the string. If they don't, start over.
Describe only what's necessary, and no more. Don't try to explain subtle differences. They will be lost on most users.
If a significant term gets repeated within a screen or block of text, find a way to use it just once.