RPM does fluctuate at lot and there can be a number of reasons.  It is interesting to understanding whether it is CTR changing or CPC (as both result in RPM changes).

This is a website or traffic performance issue.  If the website hasn't changed then this will often be cause by either the traffic being different day to day, or that traffic going to different pages.  We see this a lot on "viral" sites where the mix of pages viewed can vary a lot from day to day.

This is more common and can also be influenced by different content or traffic patterns.  If you isolate those  (by serving particular ad units/channels to particular audiences) and still see fluctuation then it is caused by demand.

AdSense prices are governed by auction and the supply or ad space and demand for it change continually to impact that auction. At a wider level this can be seen mostly clearly in the different in bid levels in December and January in western countries. This happens on a minute to minute basis at smaller levels though.

How to Increase RPM in Adsense?

It can be done in two ways.

1) Reduce the number of ads (not performing) on your blog. Of course it will reduce the earnings, but RPM will be increased.

2) Increase your earnings with same impressions.

Quick & Simple tips to improve the earnings :

1) Above the fold.

2) Don’t place all ads in same screen when your reader is reading.

3) Don’t blend too much with your theme style.

Let them know it’s a link and ad.

4) Consider to improve Search Engine traffic.

5) Block the un related ads

6) Remove the ads which is appearing in all slots. ( they are paying less)

7) Try the recommended ad size formats.

Do not try anything new with new ad sizes.

8.) Use section targeting.

9) Experiment and test, but don’t decide anything quickly.

Give minimum one week.

10) Insert high paying keywords in your blog related to content.