The Right Players
Any NFL general manager will tell you that without the talent, you won't win. In "Madden '08," there exists a nice loophole that will allow you to beef up your team roster with the best free agents on the market. Simply sign the best player available to you on the Free Agent list while in Franchise Mode. The player will demand (naturally) a huge contract, so give him his asking price but only sign him to six years with your team. This makes the salary cap decrease and frees up space to sign the second-best player and so forth, until eventually your team is packed with Pro Bowl starters.
Get Picked
Thanks to the loophole, you can also guarantee your team will land the first overall draft choice while in Franchise Mode, regardless of how well your team did the previous season. Sign every available quarterback and halfback during the free agent signing frenzy during the off-season. Trade them away to whatever team currently holds the top spot in the draft, thus automatically giving your team the first slot on Draft Day.
And Get Paid
"Madden '08" contains a special glitch that you can use in your favor to gain as much cash as possible, thus helping you sign high-dollar free agents. In GM mode for the franchise, set your team's advertising budget as high as the game allows (different amounts for different teams). Next, play a game or simulate it if you'd like, as long as you don't simulate the Monday Night Football game (which counts against you). Play until you reach the next Monday Night Football spot, then set your budget amount to $0. Play or simulate the remaining games on that week's schedule. This gives you all the free revenue from advertising/attendance without ever having to pay advertising costs.