An old and pretty safe trick for this is to put a short 2x6 under the front and another under the back wheels of your floor jack. This will leave a 1 1/2 inch gap under the jack through which you can slide a chain. Run the chain up over the frame and fasten back to itself. Now your frame can't raise up without breaking the chain and you can jack the lower control arm up as far as needed. This works fine on springs that will fit into place (like a MII) but doesn't work on long springs that won't fit up into the frame pocket and onto the spring seat in the lower A-frame, then you need a spring compressor.

Evan