1) Food Management
2) Variable Ratio Reinforcement
First you have to get pretty good recall in the first place using Continuous Reinforcement. Every recall, click and reward. Try to avoid recalling the parrot when you think it won't come. If you've already done a lot of recalls, parrot looks distracted, you know parrot doesn't want the reward, etc, don't even bother calling because it solidifies that it won't have to always come. Don't do too much other training in the same session either because that dilutes the motivation.
Try to vary the treats and use alternative rewards from time to time such as toys, petting, and attention. These are unlikely to be very effective with a cockatiel so it's up to you to figure out how well they can work. Train in a low distraction environment. Practice the bird flying from a low distraction place such as a
Parrot Training Perch. Also never show the treat to encourage recall (that's luring and the bird won't come when it doesn't see it). Hide it in your hand and even if you don't have it, pretend you have it the same way. Use a target stick if it doesn't know to come.
If you can get 5 solid flight recalls using this method (and even not any more) most training sessions, then you can apply variable ratio reinforcement. You want to have a solid history of recall preceding this so you don't end up erasing it but if you are sure the bird knows what to do, this won't undo your efforts (or only temporarily). Begin by not giving a treat randomly once out of 3-5 recalls. Just click, give the bird attention, let it catch its breath but don't give it the treat. Progressively decrease the number of times you reward out of the number of recalls but be sure to keep this random and unpredictable. You will find that the bird will recall with the same or even better reliability (if you do this right and don't advance it too quickly) rather than worse. The parrot will learn to come whenever you call for the mere possibility of getting a treat (or something good) rather than only when it wants the treat. It's more likely to want the treat if it gets rewarded less frequently as well.
For an experiment, I got Kili to fly 60 recalls (over a mile of distance) for a mere 4 seeds. Incredibly her reliability was the same as 1 treat per recall (which would have otherwise been just 4 recalls)! The point is that if the parrot can do so many recalls for training and practice (not to mention great motivation for exercise), then the likelihood it will recall to you when you just want it to come for you, the chances are much higher. If it is used to recalling on a ratio of 1 treat for 10 recalls, then the 9 times you recall it for the hell of it vs the 1 time you sneak it a treat you hid away for that random time, it will give you the chance to have reliable recall without it looking like it's all for food. Of course if you can convince your parrot to come to you merely for scratches or attention, that's even better but I think that's much harder with budgies/cockatiels. And with any parrot, there's only so many times it will come for those things. But with the presence of some element of hunger, it will recall reliably for the possibility of getting a treat as long as the hunger is still there and recalling is the only way of attaining it.