As a little overview how to avoid and stay soft:
In the day there is the psychological and the physical aspect.
Psychologically: because you want no erections anymore you should picture yourself as an asexual or more female being. Up to you to chose in what degree or combination you want this. Therefore it is important that you always pee sitting as somebody that has no penis. Always have your penis pointing down in your underwear. Cross you legs when sitting. Buy maybe panties or stockings if it enhances you not-male or female thoughts. You can for example buy and wear clothing that have a asexual or female connotation as female jeans without a zipper in front or dungarees eventually without a zipper in front. Jogging pants are also quite asexual. This is less obvious but helps!
Physically/sexually: you have to learn to your brain and body that erections do not trigger orgasms anymore. So you have to "untrain" your body. So no masturbation anymore with a hard-on. You can perfectly orgasm and ejaculate with a soft penis and that is perfectly ok, this wil come wit time. The easiest way to obtain this is by using a butplug to start with and a then dildo. So to start with always have a butpug or a dildo inserted and fuck yourself with it while you masturbate you hard penis. The trick is to stroke yourself to the point just before you orgasm and then stop stroking and stroke further with the dildo in your ass till you orgasm. So you will move the orgasm-triggerpoint to your ass. In the beginning by stroking it and later by only massaging the head of your penis. The first stage is that you will learn to your body that you can not orgasm anymore without something in your ass. In fact a plug or dildo against your prostate helps to build op the pressure in your prostate to ejaculate. If you use al the time plugs or dildo's against your prostate it will get lazy and it will be difficult to orgasm and ejaculate without something up your butt. So if you have to or want to fuck your girlfriend/boyfriend/wife it will get difficult to impossible after some time. The main purpose is to move your erogene center from your dick to somewhere else with your ass as the most effective one. Fucking your ass can give you as a reward a prostate orgasm or even better after quite some time a full body-orgasm or what they also call a sissygasm. Once you have reached that you will never want a standard orgasm again induce by stimulation of the penis. The ideal timing to have a prostate orgasm is after 7 to 10 days of not having had an orgasm. The younger you are the shorter that period can be.
One of the best other ways to cum without an hard-on is with a magic wand vibrator on your penis.
Works very well directly on a chastity cage or on a naked penis put a sheet of plastic and lube over your soft pointing downwards penis, put on top of that a towel and masshe the head of your dick through it ideally with a plug ip your ass.
If you like/practice strap-on sex with a female partner or bottom sex with another guy do not stimulate your penis or let it stroke by you top when he/her is fucking you. A good method is wearing a chastity cage when having sew with a partner. It makes a hard-on impossible but gives you also the psychological message and physical feeling that you can not get hard. It is important that you disconnect sexual or bodily pleasure when being fucked from your penis! Also be aware that the pleasure of being a soft total bottom, submissive, cuckold is found in the psychological kick and not only in a way tor orgasm. So it is not necessary in that role that you have to orgasm each time you play, giving someone else pleasure, the power pay etc. is also very rewarding. I am almost impotent but like very much the symbolical and visual aspect of a chastity cage so I wear it for example when I go to a sex party or gaysauna. It makes it very clear that I am there not to use my penis but to be available for the other tasks. Licking vagina's, sucking guys and being fucked.
At night: wear a chastity cage because you are then not conscious and you can not have a conscious control to not have erections. Use the most comfortable and discreet chastitycage avalaible. To my opinion: the Holy Trainer 5. I know it is expensive but it's very good. Containing your erection in such a mechanical way will make that your penis can not get hard by the physiological habit the body has to rise the testosteronelevel in the morning to train your erections. The principle "use it or loose it will" work. Practically it is enough to wear the cage only when sleeping. Put baby-oil on your penis and put the cage on. The first night you will wake up because of the pain of your penis that tries to get hard but after a few days your body will accommodate to that and know that it is not worth to get hard. The older you are the easiest it goes because the older you get the lower your testosterone level is and also your libido the younger you are the most difficult it is. If you get awake from the pain the easiest and fastest way to get rid of the growing erection and pain is is a cold shower or on your penis but that's annoying and you have to get up for this. The method I used was always having a glass (because it is easier to clean) buttplug next to my bed and lube to insert in my ass to divert the attention from my penis to my ass. The longer you practice this method the bigger the buttplug should be to keep the effect because you will get used to the dilating effect. To keep your sheets clean it is ideal to have pull-on incontinence briefs to pull on for this method. I know it is a lot of work... Oh, when you have to pee you will have to sit down on the toilet anyhow.
I wish you lots of pleasure!
