Personally, I have both Mining and Herbalism as my professions, and here's why
Reason:
By using both of those professions, especially at a low level you will be able to, for example, take advantage of the high copper ore prices, and say, the high prices on Khadgar's Whiskers (a herb that had high prices on my server). Now by utilizing these two (and remember every server is different) i was able to get this at level 42 (even though the picture is at level 53)
Macro:
Simply bind this to a key on your mouse (or keyboard) and press it every few seconds, it is annoying at first, but affording your epic mount at level 40 is something that is worth it (and remember, this action turns involuntary)
/castsequence Find Minerals, Find Herbs
Why not just Herbalism/Mining and Skinning?
I get asked this a lot by my guildies.
If i take skinning, that's just what everyone else on my server is doing... items that can be gotten by skinning are relatively cheaper on the AH than items from both Herbalism and Mining. So by doing this, i can come across a node/plant every few seconds and have my bags fill up like crazy. In fact, i suggest rolling a bank alt and just to post all of your items for you.
Hope This Helps