Your walkon market is going to start with marketing, like others have said. One of the big reasons why paintball fields do well is because the fields align themselves with stores, fields carry the stores flyer and the stores carry the fields flyer. The stores setup customer appreciation days, this is where it's slightly cheaper for the customer to come out and play at a field the store is aligned with, and they get to play with all the customers. I have done a few of these and it is a lot of fun. It keeps the customer coming back to the store and it keeps people going to the field. Now you have the store driving players to a particular field.
To take it a step further you then need to market outside of your current customers, the general public. Why do you think SC Village is so successful? Their pamphlet can be found at airports, restaurants, in the newspaper, etc. You get off the plane at Burbank airport and SC's pamplhet sits right next to Disneylands. They are reaching out to potential customers, and reinstated with existing customers that they are the best. All of my friends who only play 1 to 2 times a year always bring up "that place in Corona" (SC) as the first place they played. Their field also welcomes all customers, rental/walkon/tournaments and they do not force you to buy their paint. BYOP really helps people progress from rental to walkon to tournament, because unfortunately FPO really makes it difficult for players to play often. At the very least fields could require rental players to purchase x amount of paint before being able to use their own. Or forcing rental players to buy their paint. This is done at Hollywood sports where the people playing on the walkon fields must use FP, while the tournaments players on the other side of the complex can use their own paint on certain days.
There is also the opportunity for another great business here. A friend of mine runs a very successful company outside of their store business and what they do is run walkon days every other weekend. They alternate between two fields. They come out to the field, setup a tent, register players, rent out equipment, sell paint, ref their games, bbq (included in the price), tech support, raffles, co2, some times have a band, etc. Notice how this company takes care of everything and the field just provides insurancre and a place to play. The field gets a per player amount from the fees the company collects (probably about the normal amount for a non rental customer). The player is then more likely to come back to one of these two fields on a weekend that an event is not going on. This company also organizes birthday parties, corporate gatherings, etc. They make it very affordable for people to play the field gets their cut and the players keep coming back which builds the player base. This company also markets itself, handing out business cards with the event dates, these get distributed at expos, through stores, personally from the owners, etc. To give you an idea on the success of this company they were able to fund a team fully for 3 seasons, and this season instead of funding a team they purchased one of those large supair ball areans with reballs and can now travel anywhere, set it up anywhere (such as fairs, parks, etc) and introduce even more people to paintball.
It sounds like to me (this is only based on posts here) that many of the field owners are content with their rental groups that buy paint from them and come out 1-2 times a year, while they miss out on the walkon and tournament market. It should be a cycle where the field caters to all players, a player comes in as a rental player and progresses to walkon and then either stays there or moves on to tournament paintball at some point. Along the way the field owner gets money from this player more often then if they were just a rental customer. Would the field owner rather get 80-100/year from a player or 15-20/month from a player? Also logically, as a player moves up they will bring more people into the fold, their friends and family and the process starts all over with a new player.
Another one of my friends has ran two fields, one of which he used to buy his first house and another which is starting to really thrive. He's done this by being super cheap when building the field (get stuff for free wherever he can such as raildroad ties, poles, dirt, wood, etc), doing trades with teams such as a team buying a supair field to put on his property and in return the team plays for free and can run tournaments there, as well being a smart business man and marketing to customers.
Anyway, that's just an outside in perspective of what I see.
-Ron