To expand on @Martin's suggestion of using a non-admin account for day-to-day operations: you can convert your existing account to a non-admin, and save the trouble of switching to a new account:
- Create a new administrator account (in System Preferences -> Users & Groups)
- Log out of your regular account, then log in as the new admin
- Open System Preferences -> Users & Groups, and remove the admin rights for your regular account
- Restart the computer, then log in with your regular (no longer admin) account
Note that even though you aren't logged in as an admin, you can still perform administrative tasks; you'll just have to provide the admin account's name & password each time (e.g. by clicking the padlock icon in System Preferences).
You can tighten things a little more by putting some additional restrictions on software installation by tightening the Gatekeeper policy. Open System Preferences -> Security & Privacy -> General tab, click the padlock and authenticate as the admin, then change the "Allow apps downloaded from:" policy to "Mac App Store". Note that you will still be able to install other apps, but it'll require a special override (see "How to open an app from a unidentified developer and exempt it from Gatekeeper" in Apple's support article on Gatekeeper) -- which will require the admin name & password.
If you want to be really restrictive, you can even limit installations from the Mac App Store by enabling parental controls on your account (you'll have to actually log into the admin to do this), and setting limits on what apps can be purchased, installed, and run. But I'd consider this overkill, since everything in the App Store has been vetted by Apple; it's not an absolute guarantee it's not malicious, but I'd consider it pretty safe.