I would love to lucid dream more, but my ADHD precludes techniques like SSIlD and WILD that need me to concentrate and stay motionless for long periods. Any advice?
Reality checks during the day and actually asking yourself … could this be a dream?
Reality checks can definitely be a good technique if they work for you. (I’ve never had much luck with them, but I’ve heard from lots of others that use them as their main induction technique.) In conjunction with reality checks, you can also do MILD, if you don’t feel too restless repeating a mnemonic. They involve a similar mechanism (training yourself while awake to think a certain thought while asleep) so they complement each other well.
Regardless of induction techniques, the best thing you can do to help yourself lucid dream more is keep a dream journal regularly. Even very short entries will help, so don’t worry if your ADHD gets in the way of extended writing sessions.
Also, I will say that I have debilitating ADHD, and despite this WILD is my go-to technique and the one I’ve had the most success with. Obviously everyone is different, including ADHD sufferers, so what works for you may not work for me and vice versa. However, I actually think that if the manner in which you’re doing something like WILD or SSIID requires a kind of straining and difficult-feeling concentration, you may not have much success regardless of your ADHD. I’ve been practicing WILD here and there for 18 years now 
and one thing I’ve learned over that time is that I’m most likely to succeed if the process feels gentle and relaxed, not like hard work. Most of the WILDs I have these days happen on accident, which shows that it’s possible to have them without “trying” at all. 
If you want to keep trying with WILD, I have a few recommendations that might make it easier for you as an ADHD sufferer. If not, feel free to disregard the rest of this post. 
One is to only attempt WILD during WBTB, a nap, or when going to bed after a period of heavy sleep deprivation. If you’re not sleep-deprived, you’ll usually go into NREM at the beginning of sleep, and your REM cycles will be short and low-quality when they do happen. It’s not impossible to WILD under those circumstances, but it’s difficult and generally unsatisfying. By attempting WILD at the times I described, you’ll have an easier time falling asleep and the resulting dream will be more likely to be long and detailed.
Also, make sure you’re quite sleepy—sleepy enough that you could fall asleep within a couple minutes under normal circumstances. If you don’t get strong hypnagogia within a minute of closing your eyes (ideally visual, auditory, and tactile all at once), you’re likely to just keep yourself awake if you try WILD.
Lastly, when performing WILD, focus on your hypnagogia in a relaxed, gentle way. If you try to focus strongly, like you’re playing chess or waiting for someone to throw you a ball, you’ll stimulate yourself and stay awake. Instead, focus passively, like you’re watching waves come in on the ocean. Let the sights, sounds, and sensations of your hypnagogia come to you and envelop you.
It takes some practice, so don’t worry if you don’t succeed at first. If ten minutes go by and you still haven’t fallen asleep, I’d give up and just try to fall asleep like normal. It may take you several attempts before you succeed. Perseverance and keeping a dream journal go a long way.
Thanks for taking the time to write all of that. In case you are wondering, I already do MILD often and keep a dream journal. It’s the WBTB-based techniques that have given me trouble since I tend to drift off in the middle of them or the concentration keeps me up for hours.
You are totally right when you say that I’m straining myself doing the WILD and SSILD. I know they’re good techniques that could work for me, but it feels like concentrating is like holding up a heavy weight that I can’t carry for more than a minute. If I could lighten the load, maybe I’d be able to focus for more time. You say it should be natural and easy, but that’s easier said than done. As for the hypnagogia, I will make sure to stay relax when the visions appear if I can. Before I try the WILD again tomorrow morning, do you have any advice to keep me on task without straining my focus?
Sure thing! 
I think when you do WBTB, it might help if you don’t wake yourself up too much. For me, if I’m doing WBTB and I get up for a while and read a book or something like that, my mind will start to race when I go back to bed and I’ll really struggle to fall asleep again. If you’re like this too, you might try staying in bed and trying to fall asleep again right after you wake up. It’s possible that you’ll just fall asleep again without managing to do WILD, but if you tend to keep yourself up for hours trying to do it I think it’s better to try it in states where you’re very sleepy. I think you’re much more likely to succeed in that state than if you’re awake enough to keep yourself up just by thinking. If you do get up, maybe just get up for a couple minutes or so.
Also, as far as staying on task without straining, I guess what I’m saying is that it’s good to let your mind wander to a certain degree. If you let your mind wander completely, of course, you’ll just fall asleep like normal. But if you’re having trouble with “trying too hard” and keeping yourself up, I think it’s worth approaching it almost exactly like you’re falling asleep like normal and just pay a very little bit of attention to what’s happening. You can experiment with how much you actively focus on your hypnagogia vs being so passive that you just fall asleep ordinarily; with practice, you may find a “sweet spot” that lets you pass into a WILD quickly. At first, this might sound kind of confusing, or feel awkward–like, it might seem that you can only focus very actively or not focus at all. With repeated attempts I think you start get a sense of the finer gradations of hypnagogia focusing, so if it seems confusing at first, just keep trying.
It may take some patience; I struggled a lot and had lots of false attempts before I got the hang of it. There was a long time early on when I thought WILD was impossible for me. If you’re keeping a journal and doing MILD and so on, I think it’s probably only a matter of time before you succeed, so don’t lose hope. 
If it doesn’t work tomorrow morning, just try again the next time you get the chance.