I agree with Shadroch for the most part about dollar cost averaging, which is what you do when you buy at regular intervals for generally the same amount. That way, the low points and high points get averaged out, but we are all assuming that the overall value will be trending higher in the long-run. This avoids the trap of trying to time the market.
However, I do buy some bigger blocks when those big cap dividend payers are "undervalued" in my mind. Over the last 5 or so months, I have been diverting more free cash into 3 of my "favorites". Fortunately, earnings of those companies have turned out strong although the overall economy is still in a rut (housing and credit related).
If you are interested in monthly buys into big caps, you can always look for those that allow you to invest directly. You invest directly with the company and their financial company, allowing you to have direct debit transactions with no or little (like $1 per transaction) fees. The big downfall with this is that you can't time the buys, and it takes several days if you do wish to sell some or all of your position. However, this approach should be taken with a long-term build and hold focus, so you aren't looking to make a fire sale. These should be big old stable companies anyways (in my eyes).
FYI:
Large cap > $10 billion
Mid cap $1-$10 billion
Small cap basically < $1 billion
Market cap (-italization) can be determined by multiplying the number of shares by the share value.
good luck