In Gmail you have to type in a label, click it to accept it, then click labels again to type in another one...etc. etc. for all the labels you want to apply.
Is there a way to apply multiple in less steps -- for example the way you might apply multiple tags using a comma?
In addition to the good options Ian offered of dragging labels to the emails and using the list with the mouse as a multi-selector, there is also a way to speek this up with the keyboard.
While typing labels you can accept the current one by pressing enter instead of switching to the mouse to click. You do NOT have to even type the full label, as you type the nearest match will be accepted on enter. If you have keyboard shortcuts turned on in your gmail you can label any open message or selection of multiple messages by pressing the l key. If your labels are unique enough that they don't usually share a first letter, this get's all labeling operations down to three key strokes. l > X > Return where X is the first letter of your label. L enters label mode, start typing until you have a label match, then enter to accept. Repeat to add another label by hitting L again.
You can drag them from where they appear under the "compose mail" button, onto a message, whether it's an open email or just a line in your inbox.
Alternatively, don't type anything in the filter box. Just select all the check-boxes for the labels you want to add (maybe you'll have to scroll). As soon as you check the first one, the "apply" button will appear at the bottom of the list and clicking this will apply all labels that are currently checked.
