Is there a way to order buttons in launcher by frequency of use? Have around 40 items and small screen size to view all of them. Did a search on software center, search engines but nothing about it.
For your case, since you have a small screen and a lot of items to put in the launcher, I would first suggest the following:
By using a tool like Drawers you can save a lot of space if you group apps with similar usage (Drawing apps, sound apps, office apps...)
To install, copy/paste the following lines in the terminal:
sudo add-apt-repository ppa:ian-berke/ppa-drawers
sudo apt-get update
sudo apt-get install drawers
To use is easy, create a "Drawer", put it in the launcher and just drag from the Dash the items you wish to have in that specific drawer.
Manually put in the top, the most used apps. Even though is manual, at least it will save you more time since you have the most used ones in the top.
Dash remembers. Open Dash an by typing the first letters of an app you normally use or rarely use it will pop in. You don't even have to write the whole name. If it appears as the first one, just press Enter. I know is not a solution but since what you are looking is to save you some time, pressing the SUPER and typing 3 or 4 letters is faster than looking for the app in the Launcher sometimes.
Post the idea in http://brainstorm.ubuntu.com/ so it gets picked up and maybe included in a future version of Ubuntu.