Getting to the next level, whether 3.0, 3.5, or higher, isn't easy. The answer is practice, of course, but just getting out to the court every day isn't enough. You must practice with a goal in mind, and you must know HOW to practice to achieve that goal. One "simple" way to get better is to concentrate on the skills that are missing from the level above your current level (based on these criteria as noted by the USAPA). ​
Let's say You haven't been playing pickleball that long and you want to get to be a 4.5 player. What do you need to do? What are the major differences between 3.0 and 4.5?
The table below (info from the USAPA website) lists these definitions:

I highlighted two items in the table that popped out at me: CONSISTENCY and STRATEGY. Other elements are important, of course, but by the time a player has achieved 3.0-level skills, he or she knows how to do a lot of things—just not how to do them well, or consistently. What that player doesn't really know yet is the strategy that goes along with what is known: when to hit a dink instead of a drive, when to break out of dinking, and so on. That's the strategy bit, and learning is not all—it's also being able to know how to do it automatically and not have to think about it.

If you've read my posts before, you know I stress PRACTICE, and not just aimless hitting back and forth over the net. It's important to have a goal—one that's reachable and measured. Like consistently hitting that forehand that sometimes goes out of bounds or into the net.

Get a friend to work with you (or better yet, take classes) and make sure 1) you're doing the stroke correctly before you continue to learn it wrong; 2) concentrate your focus not on everything about that shot but rather small parts of it: hitting it at about the same height and to roughly the same spot on the court.

Once you have mastered that, move the shot to other areas. When that's easy, work on your backhand and do the same thing. All of this takes time, but if you do this practice consistently (couldn't resist putting that in), your game will improve.

Take things slowly and methodically, knowing it took a while to get from novice to 3.0. With patience and some help from your friends, you'll get there.

Earlier this year, I spoke with a group of pickleball enthusiasts at a local community. They are a new club, having only built their courts about a year ago (and they already have over a thousand members!). One of their questions had to do with how the clubs of which I have been a member handled "ranking" their players into levels for the purpose of playing on ladders, taking part in ranked training, and entering tournaments. It so happened, I'd just seen a post on Facebook that defined skill levels this way:

This list is not far off the mark.

Most clubs judge a player's ability by watching him or her during a game, and the criteria don't vary a lot from one club to another (in fact, if you search online for "pickleball skill level definitions," as I did," you'll see many documents listing the same things). 

I looked at a number of the online documents and compared their lists of required skills for the levels from 1.0 to 5.0. I've posted my compilation of the most common criteria on a separate page (click here). 

Some players don't like skill levels. Certainly, these levels can cause problems when, say, a husband and wife take their first pickleball lesson, hoping to play together, and a short time later, one of them has progressed to a 3.0 level, while the other is still a 2.0. And then there's the argument that the freedom to "play up" helps you increase your skills faster. That's true, but mixed-level play often only benefits the weaker player (not to say you can't have fun playing with anyone, no matter their level). If skill improvement is the goal, it's best for everyone if players are at the same level.

It's hard to assess our own skills, and quite often we think (or hope) we play better than we really do. Having an impartial judge point out the things you need to work on to achieve the next level is helpful for structuring your practice AND it also tells you what you're doing well. It's important to take stock of this, too, as you strive to become a better player.

Taking a skill-level assessment in stride reminds me of what happens when I get a good edit for my writing. I come away with lots of things to work on, at the same time that I know the rest of my work is okay, and I can build on that foundation. 

So take an honest look at my composite of the skill level documents and figure out what you need to work on to get to the next level.