Spirit Airlines "No Frills" policy is probably one of the most strict in the airline industry. Anyone who has flown spirit knows that they charge for everything. These are to offset the cost of extreme low fares. Bags are the the largest revenue source for Spirit flights outside of the fare cost.

Bags are also the biggest reason passengers get upset with the airline. Unless you pay for a check in or carry on, you are limited to a "Personal Item". Be careful, if you get caught with an over sized item, you can be charged $100 at the gate. Even if you are traveling on a $16 ticket.

The following trick works. Even domestically and internationally. I have done this to and fromBoston, Ft. Lauderdale, Chicago, even to/from Colombia. I would say easily 50+ times in the past 2 years.

Here is the trick.

1. Do not pay for bags (Carry-on or Check in)

This is the main point of the hack. Enjoy your cheap ticket and fly with whatever you can fit in one bag without paying extra.

2. Take any size duffel bag. I mean any size. No bags with wheels!

Spirit airlines site quotes that

"A personal item may not exceed 16 inches by 14 inches by 12 inches (40 cm x 35 cm x 30 cm).  If it does you will be charged the appropriate carry-on charge."

You do not have to follow this rule if you bring a bag with no wheels. This is not a rule, its a suggestion that works most of the time. Do not call your bag a carry on. To anyone.

3. Do not talk to anyone that works at Spirit

Avoid contact at all times with anyone working at Spirit. They can easily identify you having an "illegal bag" and you will be forced to pay fees. Do not even go to the check in counter. Print your boarding pass at home or at the kiosk. If a Spirit attendant is standing and staring at bags, turn the other way. This person's job at that moment is specifically to catch people.

4. When you are in line to board, blend in

If you got to this point, you are almost to (bag) freedom. This is the last step. Also the most crucial. The boarding process might be the most scariest because it comes with the highest penalty ($100 fee). The trick here is to keep your over sized duffel behind your back. Rest assure that the Spirit staff member is really trying to just get everyone on board as their priority. If you blend in with a group of people, he or she will scan your ticket without noticing anything. They normally catch people who are rolling items on board because those are easy to spot. 

Congratulations! You saved even more money on Spirit.

Caution: This is based on personal experience and analysis of Spirit Airlines. The following hack is not 100% successful, but it is pretty close. You are responsible for your risk taking.