Hi Jack,
A twice a day mediation practice will certainly most helpful to make the desire less strong, so it loses it's grip on you a little every day. If you can keep it up diligently, you've already come a long way. There are several things additional things you can do to aid yourself in overcoming this addiction. First there are some physical maneuvers that help with changing the flow of your life energy from down and out to upwards along the spinal nerve. If you are trained in asana, Sirsasana (headstand) is great to do twice a day and when the urge of masturbating becomes strong. Only perform this asana if you have learned this from and practiced under a qualified teacher! Applying
Mulabandha in various practices may help, as well
Spinal Breathing. If you already integrated these aspects in your practice, I would recommend to introduce
Yoni Mudra Kumbhaka, since in my experience this practice strongly pushes the energy from the lower centers upwards.
Second, if you have an interest to work with your desire on the mental plane, it might be worthwhile to have a look at the way the yogis view the faculties of the mind (known as
Antahkarana), so you learn where desires are coming from, and what phases a desire goes through before it is manifested in the outer world. It's fascinating stuff, that may seem a bit daunting at the beginning, but I find it a lot of fun to play around with and experienced it as a great tool to overcome addictions and strong habit patterns.
In a nutshell: ideally, it is Buddhi (the intellect, the one who discriminates) who gives orders to Manas (the faculty that governs the senses i.o.w. imports and exports information and actions). However, most of the time it is the impressions stored in Chitta (our "memory bank", where the samskaras reside) that surface and order Manas to perform certain actions. Ahamkara (the Yogis sense of ego, the I-maker, the one that makes you seem an individual separate form others) identifies with this action every time you perform it (I like this! or I don't like this!), so you become more attached to the action every time you perform it.
By doing an action like masturbation over and over and over again, this Samskara becomes so active and strong that it shouts a lot louder then Buddhi. Since Manas obeys the one who shouts the loudest you act from habit (samskara) instead of choice (intellect/free will). A lot can be gained by training your mind to listen to Buddhi, and train Buddhi to become sharp so it makes the right decisions. In this case this would mean that you make a very firm decision that you don't want to indulge in masturbation anymore, make it so firm it is simply not an option. When mind starts to argue with you never the less (o come on, it feels so nice and you deserve it) just tell your mind that this line of thought is not useful and that the mind can argue what it wants but you will not order Manas to act out this desire. Believe it or not, there comes a time that mind simply gives up the arguing. The Samskara will still be there, but every time you do not act on it and let it go, it becomes less and less strong. Ultimately, it will 'go to sleep' i.e. go from an active desire to a latent one in your subconscious.
Third: It can also be useful to simply ask yourself why you masturbate. The answer may surprise you. It can help you to see this urge for what it is, and reminding yourself of that when you feel the desire can weaken it.
I'm sure there are many more ways to deal with this problem so I hope that more will chime in to share their experiences. The above is what works for me, but it is important that you find an approach that suits you which might be something completely different. All the best to you!