[joezydeco]

One way is to go back to your original company and say you're leaving. You then either sit back and see if they offer you more money to stay, or you explicitly tell them "NewCo is offering $X" and see if they match or beat that.

This move, while very gutsy and sometimes very effective, only really works in an industry where you're in high demand and/or you're essential to the operation of your current employer.

But, in either case, there's no guarantee. Your old boss might certainly counter-offer and give you the higher salary, but only long enough to find a replacement employee at (or even less than) your old salary. Then your salary very abruptly drops to $0.