(1) Apple Macintosh

For the Apple Macintosh (under OS X) you will quickly get used to this simple keystroke combination to type brèves: Option + b, followed by the base letter.
You would then type:

ŏ —> Option + b, and then o
ŭ —> Option + b, and then u
Ŏ —> Option + b, and then O
Ŭ —> Option + b, and then U

For the romanization of Japanese (Hepburn system, for the macron), on a Mac, this is just as easy—use Option + a:

ō —> Option + a, and then o
ū —> Option + a, and then u
Ō —> Option + a, and then O
Ū —> Option + a, and then U

Important: For this to work on a Mac you need to choose the keyboard driver "U.S. Extended" (System Preferences —> Language & Text —> Input Sources).

A customized version of the "U.S. Extended" driver that allows you to also use the Option + z combination ("z" is right above the Option key, that makes it easier to type) can be downloaded here: http://koreanstudies.com/notebook/MacOSX-US-Ext-Modified.zip (for all Mac OS X versions). The folder includes README files.

In case you are working with a non-U.S. keyboard layout, please consider installing one of the 11 Mac keyboard drivers (German, French, Dutch, etc.) I created for this purpose. You can download them here: http://koreanstudies.com/notebook/MacOSX-KJ-Drivers.zip (for Mac OS X 10.5 or later). The decompressed folder includes a README file with further explanations. Or read my forum post about the drivers (reposted 09/17/2015).

For other accents and all kind of characters you seldom use, so you would not so easily remember them, simply install the small application "PopChar" (http://www.ergonis.com/products/popcharx/).


(2) Windows (XP - 7 - 8 - 10)

The most popular little tool for this purpose seems "AutoHotkey" (http://www.autohotkey.com), a free tool that allows you to easily create shortcuts for the accents you often use, and those shortcuts then work for all applications. See the "by the way" EXAMPLE at: http://www.autohotkey.com/docs/Tutorial.htm#Send

A cleaner alternative is to just create your own keyboard driver for Windows XP, Windows Vista, or Windows 7, using the Microsoft Keyboard Layout Creator (MSKLC). Instead of using e.g. "US International" or "French" you would create your own keyboard driver based on whatever standard driver works best for you, and then just change certain key assignments the way you need them; create your own driver, call it "US Breves," "French-Breves" or whatever you want. Info and download link: http://msdn.microsoft.com/en-us/goglobal/bb964665.aspx

Using the above program by Microsoft I have created two sets (for Windows XP and Windows 7) of keyboard drivers that you can download and install: U.S. and British keyboard layouts, German, Dutch, French, Italian, and others—all will let you easily type brèves and macrons. Instructions can be found in the README text included with the files or in my forum post (reposted 09/17/2015).
Download links:
Windows 7 and Windows 10:
Windows XP:

Windows 8 --> See here!

U p d a t e / US driver (July 24, 2016):

As was pointed out to me, when using the US keyboard driver one automatically inserts an accent aigu (é, ó ...) while the intention is to type a simple apostrophe (if a vocal follows that apostrophe without space). FOR EXAMPLE, if you type
   t PLUS ' PLUS o
which should result in t'o ... e.g. to represent the Korean 토, you actually end up seeing . Here is a new version of the US driver where the "dead key" function of the apostrophe has been removed. (Just switch back to the original keyboard driver if you need to type é, ó, etc.)
Newly customized US driver, Windows 7/8/10:

Frank Hoffmann