You’re not going to let that stubborn, stuck screw win, are you? Forcing the screwdriver with muscle power alone will likely strip the slot in the screw head. You need brain power too. Clamp some Vise-grips or an adjustable wrench onto the hexagonal shank of the screwdriver. The wrench gives you more leverage to turn the driver; use your other hand to push the tip firmly into the slot. All your screwdrivers have round shanks? Move the wrench up to the handle, but be careful not to twist so aggressively that you break it.
If it’s a seized nut you’re arguing with, grab some spray lubricant (WD-40, for example) and your bag of hammers. First douse the nut with lubricant and let it soak for a few minutes. Hold a hammer against one flat side of the nut, and with a second hammer, give the opposite side a firm, persuasive blow—something between a tap and a whack, hard enough to convince the nut to give up the fight.