Understanding your audience
By spending a little time up front identifying and indeed segmenting your audience will provide significant results.
For example, if you are targeting a technical audience your communication is going to be considerably different to those in advertising.
Points to Consider:
Subject - Which subjects are they interested in? This is not to be confused with subject lines. (Another subject all together.)
Tone - What tone to use for your communication? It is informal and friendly or corporate and professional.
Creative - How are they influenced by creative design? Can you be bold and push the boundaries or do you need to be conservative.
Incentives - What incentives do you try? Again the effectiveness of incentives will largely depend on how relevant they are to the audience.
Final thought, understand them first and be relevant!