Getting into an air tanker pilot gig is essentially a numbers game. Here's some background: First, let's assume you'll start your career as the copilot of a two-man crew. On average, there are 44 copilot slots in the entire United States air tanker fleet. These estimates don't take into account the relief crews that might be recruited during a really crazy fire season. We should also note that C-130s add a flight engineer, since that aircraft's size increases its operational complexity.
Next, understand that even if all 44 slots opened up every year, that's not a lot of openings. Now realize that the aerial firefighting industry has an excruciatingly slow attrition pace, since pilots who enter the industry often don't leave for years. This trend results in pretty slim pickings for copilot seats.
However, we can suggest a strategy that might help improve your odds: Make yourself a superior candidate. Develop some additional technical or aviation skills, such as an Airframe and Power Plant Certification (commonly known as an Aircraft Mechanic certification). This will give you some real brownie points, as your employer knows you can legally fix problems with the aircraft you fly. That will save him time and money.
Also consider working as a crop duster, which will provide you with near-to-ground flying experience. Since some firefighting aircraft are converted crop dusters, you'll have a slightly gentler learning curve. Working as a flight instructor will provide you with flight planning and quick-reaction skills, since you'll always be looking for a bailout strategy if your student does something incredibly unwise. Finally, consider a stint as a ground-based firefighter, especially a wildland firefighter. Learning how the ground crews operate will help you better support them from the air.
Now that you've gained all these valuable skills, how can you use them to your advantage? Consider that most aerial firefighting companies forecast their staffing needs during their downtimes, typically November through April. In fact, they might even be sketching on napkins while they sip margaritas somewhere in the Caribbean in January. Get on their radar by sending your résumé, politely following up (without being too aggressive), and keeping the fire season timeline in mind. Once the companies staff up, they'll probably start training in January or February. When the fire season begins, each company should ideally have a team of crack aerial firefighters (including you) to tackle that season's fire emergencies.