Tight control over the finances, keep wages low, dont spend any money on transfer fees, sign free agents and sell them after 6-12months, being successful helps as it brings money in
Sell players who you think you could live without, i did it at Everton.
Club are paying off a huge debt so i won the Euro Cup and sold unhappy players like Barkley and Baines aswell as fringe players like Oviedo and Naismith.
Then try for free transfers on lower wages if possible.
Never spend more than what your balance displays even if your budget is larger!