Being able to chance or cancel flights and re-use the money is addictive. It is also fairly complicated to do well, but you have FTers here to help. You need to keep a spreadsheet that includes the expiration date and dollar amount for each record locator.
Absolutely. Instead of the logical 'spreadsheet' method, I use the illogical 'paperclip' method. Just last week, I canceled a flight and calmly went into 'look at my travel funds' and found that my new travel funds expired in 6 weeks. Since I am not flying WN as much, I had to scramble to not loose those funds.