We use an auction dynasty system.
Our year 1, we held an auction to fill 53 man rosters for 10 teams (we start 11 IDP's each week). $1000 salary cap, minimum bid was $1 with bids in $1 increments. It took a while. But each successive year's auction is shorter as you are only bidding on players that are free agents and not on rosters because...
After each year's auction is completed, owners have to assign a contract length for each player he just won in the auction. Contracts are for 1 year up to 5 years. But each successive year under a contract has a built in salary increase. So the year 2 salary is actually the year 1 salary + 5%. The year 3 salary is the year 2 salary + 10%, the year 4 is the year 3 + 15%, the year 5 is the year 4 + 20%. So basically, if you sign a player to a long term contract, you can end up paying him almost 160% of his original salary. So you have to decide if you are going to opt for high turnover, high liquidity to capitalize on emerging/declining players each year or for low turnover/low liquidity and bet that your instincts for the long term are valid. But you can also opt for mix by signing a pre-emergence RB like Ray Rice to a cheaper long term contract while also signing someone like Westbrook to a higher priced short term contract. With this sytem, every player has some value!
Some players end up not being worth what you bought them for (or have them signed to) or you may simply decide you can do better for cheaper in the free agent market or in the upcoming year's auction. So you part ways by cutting a player before his contract is up and get some cap money back. But you take a cap hit off your $1000 cap basically calculated as his final salary less starting salary if you cut a player under a multi-year contract. Depending on where he is in his contract, you can sometimes spread that hit over two seasons.
We've also instituted a rookie draft each spring following the NFL draft where you can draft a rookie as a matter of right for a pre-calculated price (determined as a % of the top salaries for his position under contract in the league) depending on the draft slot he is chosen in or, if you think that this draft price is too high, you can elect to have that rookie auctioned instead where the other owners can bid on him as well. This gives you the option to take the guy uncontested if you think he is worth it, but yet if you can't find a guy who is worth the draft slot price for his position, you can let the market determine his value but risk losing him to another owner.
We also have a franchise tag system that runs pretty close to the NFL version. You can extend for 1 year the contract of one player whose contract has ended but his salary will either go up by 20% or be the average of the top 5 played players at his position, whichever is higher. However, if you tag him, other owners get to bid on him IF they are willing to exceed his franchise tag salary. If they do, you then can agree to match that price or let him go to the bidding owner in return for some rookie draft picks.
Free agents can be added in the season by a blind bid system but they are 1 year contracts ending with the season. They can be franchised, however.
We love our system. You can get any player you want assuming you are willing to pay for him. You can keep him for a good portion of his career, assuming you are willing to pay for him (you can successively franchise the same player). The salary cap and the roster sizes though do a good job of encouraging roster turn over, so every year's auction has some good talent returning to the pool that owners are licking their lips over. The cap hits keep someone from recklessly signing everyone to long term contracts...it sucks to go into an auction with $100 in dead cap space when everyone else has more money to spend. You'd be surprised how the studs of the league sometimes end up back in the pool because their salaries get so high.
Your bad roster moves usually end up as cap hits, which only last a year or two, so you don't have to sit back and know you are going to suck for another 5 years. And you can still rebuild your team within a year by trades and shrewd moves in the auction. But with shrewd moves and an eye for young talent, you can keep your team on top as a dysnasty for years as well.
If you want more details, PM me. To me, this system has it all.