look at the logarithmic chart for the past couple of years at
http://bitcoincharts.com/Try drawing a straight line somewhere "fair" between the highs and the lows. Choose a few points on this line from the time when you will have your money ready. Check the corresponding prices.
Depending on how long you will stay in for and also the market conditions at the time you might decide to give yourself some leeway with the actual price you will pay: a lot of positive news coverage / near-future events for example might make you may decide to buy in a little higher than the line.
In my humble opinion this method will work for most people looking at investing for 6 months to several years from now. Some might disagree with me, saying that a few months is too short-term, but given the current situation if you buy in between the extremes at some point now I think you will soon be sitting pretty.