To properly input (Hepburn) rōmaji, one should use a macron (ā, ē, ū, ī, ō) to indicate long vowels. Since those letters are not usually found on any keyboard, it'd be nice if they were part of the toolbar above the question edit field so they can be inserted with a single click or at least copy-paste.
An integrated solution inside StackExchange would be swell, but until then, there are a few other solutions you could use:
Following Boaz’s nice answer, here is another way which you may be able to use.
If you use a Japanese IME (a computer program used to enter Japanese text), you may be able to use also for these rōmaji symbols. At least in Microsoft Office IME 2010 (for Japanese text), if you enter Shift+A and try to convert it to kanji, it shows a candidate labeled “記号…,” which expands to various symbols including ā and Ā.
If your keyboard does not have a 変換 key, it might be tricky to “try to convert it to kanji” because the Space key simply enters a space character after an alphabet letter by default. I cleared the checkbox “英数モードでは Space キーでスペースを入力する” in the property to make the Space key always try to convert text to kanji (see e.g. the steps with screenshorts on the support website of Panasonic), but the precise setting for this may depend on the program and the version of your IME.