A keyword's CTR is a strong indicator of its relevance to the user and the overall success of the keyword. For example, a well targeted keyword that shows a similarly targeted ad is more likely to have a higher CTR than a general keyword with non-specific ad text. The more your keywords and ads relate to each other and to your business, the more likely a user is to click on your ad after searching on your keyword phrase.

CTR would increase if adverts were...
1: Better targeted.
2: Better quality landing pages. 

There has been days years ago when people reported CTRs like 8 and even 12 percent. This can happen provided the ads are very relevant.use to be a day when someone would get a 10% ctr. However, since people are getting smarter and really "catching on", your getting more competition. Therefore, lowering your ctr.

To tell you the truth - not sure how you can do to raise your ctr. To a certain degree we found that working on increasing CTR actually decreases your revenue. 
 
We did extensive testing with the CPA program when it was alive and found conversions to be the highest on our websites when the ads did not match the sites color schema and were clearly ads in off colors. CTR dropped like a rock but conversions picked up substantially.

This gets back to the continued push on Googles part to decrease CTR(accidental clicks) to increase conversions for advertisers. My suggestion is to place your ads in typical areas that are not to obtrusive and to go non matching.

We painfully made this decision a few months ago and after the initial loss in pay we are now making more money then ever due to substantially increasing EPC.

Less Clicks = Higher Conversions = Increased Advertiser Demand for your Inventory.

Now if your traffic is junk to begin with then you really are in a pickle because the increase you gain in CTR will just be smartpriced out. 

Advice from a guy who earns his living on the web 

Try 336x280. I have tried them all and 336x280 works better then most. 7 years in the business, I earn my living from my websites. Totally free advice!