A much safer/easier approach is to buy as often or whenever you can afford to buy and be in it for the long term. You need to only buy when the numbers stack up (This bit is important as it would help even if you did not time the entry in the market correctly). This strategy has been tested/proven and the risks can be managed.
Cheers,
Oracle.
that's the excuse i will use if the prices go up instead of down during next half a year and I have to buy at increased prices compared to now