I can only share what has worked for me. But really you will have to experiment on your own. Whats working best for me right now is only having one text ad unit. Sometimes the large rectangle works well under the title above the fold. Sometimes a small square with the text wrapped around it works best. Those are the two that are working the best for me right now.
Ad placement and blending/contrasting the ad with the contents depending on the niche will help you increase the CTR. However, the quality of traffic too plays a vital role in deciding the CTR. While search engine traffic increases it, traffic from social bookmarking sites will do little to improve it.
Try to change ad colors, and try different ad placements. Analyze the performance, and go with the best one.
Ad placement is the key. Regular monitoring of what works and what doesn't is also important. Test, test, test.