I can suggest just practicing more frequently, nog necisarily more intensively.
Start each day with a set (50% of your max). Do another set when you get home from work and another before turning in.
When you're on a Gym day, add sets of different kinds of pressups. I used to do a circuit session that went something like,
"Incline Pressups, Situps, Close Grip Pressups, Squats, Wide Grip Pressups, Regular Pressups, Step Ups, Explosive Pressups", when I was working on my upper body.
When you come measure your progress, try taking a break every ten pressups for a few seconds. You don't feel like you need it after 10, maybe after 20, but the first two breaks will be getting valuable oxygen into your arms, which will make 30, 40 and perhaps 50 feel much easier.
Hope this helps. I'm not a PTI or anything. This has just what I found to work.