Similar to others, I try not to spend miles if I have a shot at status. In practice, I only spend miles on short, expensive trips or mid-cons on holiday weekends.

My personal rules of thumb are to spend miles:

If a mid-con ticket is more than $450;
If a trans-con ticket is more than $550;
If a HNL ticket is more than $850;
If an international ticket... well, never. I would always pay and upgrade. Some day I will spring for a *A F ticket. Some day.

The factors that go into the decision to spend the miles are:
1) Savings on the ticket cost.
2) Opportunity cost of 2xRDM + EQM + BIS (CR1/e-500-earning).
3) Cost of 25K miles + taxes and fees.
4) Lost chance of upgrading.
5) Usefulness of flexibility.

As a 1K, I am enjoying the freedom of no award fees. I can book a ticket with miles months in advance and, if the fares drop below my criterion, I can buy a ticket and redeposit the miles for free.