The Mossberg is my favorite gun to ABUSE..er, break in. It loves it. A simple blued 500 is sooo much fun. My method for breaking in a pump shotgun varies slightly depending on the gun, but here's what I like for a Mossberg 500:
Get 500 rounds of Federal 7.5 Lead shot at Walmart (23 dollars for a hundred at Walmart) and a couple boxes of the cheapest 2 and 3//4" buckshot you can find. I wouldn't clean or OIL the gun before you shoot it UNLESS you don't plan to shoot it for awhile. I think the built up carbon/powder what have you that may be in there from test firing it at the factory will only help work in the action even more than the workout you're about to give it.
Take the gun out and shoot ALL the low-baserounds. Don't baby the gun. Rack with authority. rapid fire it. Don't let it cool until you absolutely have to. Take it to the cleaners basically. Put it through it's paces. Make a day of it. You probably won't have any problems with it, but if you do, don't panic. It's more than likely a simplistic fix and a blessing because diagnosing it and fixing it will make you a better handler of it in general. When you're done with the low-base shells, try the buckshot on for size, but shoot at a target you can clearly see and doesn't have holes in it so you can get a feel for the way buckshot handles and spreads and recoils.
When you get home, disassemble it and clean it good. Scrub it. When it's clean, lightly oil it. Even take the stock off and clean back there and lightly oil it as well. I like Birchwood Casey Gun Scrubber for cleaning and Remington Remoil or Outers for lubricant, but your mileage may vary. Any good solvent and gun oil will be alright. From there on out clean the barrel after every use and deepclean and lube the gun after every 500 rounds or so or if storing for an extended period keep it clean and lubed.This is how I like to break ina Mossberg 500 and in my opinion you just bought pound for pound, dollar for dollar the best common pump shotgun available all things considered. I will always have a special place in my heart for Mossberg shotguns, even if I do admire others as well.