...WD-40 will work as a temp fix, but it will displace other lubricants and then dry up. Graphite is great.... until it gets wet. Houdini is what we use on our secure, outdoor locks if they start to get nasty..... best stuff I've ever used. A can of that will last you years and years, and it will pay for itself 100X..... works great on any tumbler-style lock.
But if you've been bashing on your key for a bit, you may have already caused some damage.... just be prepared to repair/replace the cylinder if it's gone that far. Also, once corrosion has set in bad enough to start eating metal you'll be in for the repair in short time, no matter what fancy lube you use.