If you are routinely rolling back while attempting a static mount, you can practice mounting on a resistant surface, such as grass, or you can mount on a slight downhill. Over time, these scaffolds can be removed.
I attribute most of the unintentional rollback mounts on my 24" to a failure to get my center of gravity forward enough during the mount. If your body is too far back during the mount, that will necessitate doing 1. a rollback mount, 2. needing to push the wheel forward while mounting, or 3. doing a weightless step on the first pedal. On a bigger wheel, one of these techniques may be necessary, but for a tall enough adult on a small enough wheel, they are not required.
Based on my own experience, some of boilerplate advice given to beginners, while it may help them succeed at riding, is crappy when applied to mounting.
For example, holding out the arms for balance. I think it’s better to secure the saddle with one or both hands at the beginning of the mount. If the mount fails, the rider will be able to guide the direction of the dismount and keep the uni from shooting out the front or back, making it safer. Hands can be thrown up a second or two after the mount. This form of mounting, while it may result in far fewer actual rides following the mount, simplifies the physics of the mount by stabilizing the rider and unicycle.
And looking straight ahead. Most of my mounts involve looking straight down at the hub. If you are looking straight ahead, you will also tend to be more upright. How are you going to get your center of gravity more forward? Some riders have described a disorientation while looking straight down at the ground. Like anything else, it takes practice.
And sitting up straight. Mounting with a straight back puts the rider too far behind the hub.
My recommendations for any beginner who wants to improve: Get a 20" and work on mounts. After reading this forum for seven years, I’ve come across a fair number of novice riders who felt good about their riding but bad about their mounts. For me, practicing mounting has been much more than a “means to an end” (riding); it has unlocked a lot of other techniques.