1. Objectives: What are you trying to achieve? This has to be the start point and is often given fleeting thought. For each and every communication you need to think what do I want my target audience to do as a result. Then you can ask which channels could help me do it. If you are building relationships then online/ off line editorial and email marketing (along with a raft of other channels) will be key.
2. Target audience reach: Whichever channel you use, it needs to be able to get to as many of your target audience as you need to get to achieve your objectives (and within your budget). Some channels lend themselves to certain age groups, demographics and needstates. If you are targeting youth then mobile and social media are a must. If you are selling landscaping ornaments to the over 60's then print is a very relevant channel.
3. Budget: As mentioned above. If you have no marketing budget then email marketing and social media are going to be must haves. But even if you have a large budget you still need to make sure you are achieving the above, in order to maximise the efficiency of your spend.You need to look at all the options and work out what is the most cost effective way of reaching your target audience and achieving your objective. You dont need to get into complex ROI's to get an idea. Look at how many of your target audience will be exposed to your communication, how well the channel helps you achieve your objective (give it a score) and then work out the cost per communication hit.
4. Creative Content: And lastly, perhaps most importantly, the effectiveness of a channel is directly related to the quality of its content. Blogging, without interesting, engaging, value added content won't get you very far. Emails that spam or sell too much will not build relationships. Print ads that are bland, uncreative and don't inspire desire will fail, regardless of how much money is thrown at them!