As a student pilot, I sometime forget tasks such as changing radio frequency after reaching a location or switching fuel tank in long navigation.
These tasks happen in specific time or location and may not be in checklists
Make your own checklists
While the manufacturer provided check lists are a good starting point and generally cover the operation of the aircraft well the FAA does not forbid you from extending them. If you have a few more steps you feel are worthy of a checklist write up and print your own.
If you are switching tanks it sounds like you are flying a piper. Are you starting a time right before takeoff? If not you should be and you should always be changing tanks around the 30 minute mark. Another good rule of thumb with tank changes is to always execute them when near or overflying a field. Waiting an extra 5 minutes or switching 3 minutes early if their is a field below you is always a bit safer (should the engine cut out). Remember, for the pilot, a flight is not a journey from point A to point B it’s a series of maneuvers from one potential emergency landing spot to the next.
As a student pilot you are also still deloping habits which will be honed over time. You should debrief either with an instructor or do a self review after every flight. What went well what didn’t, what can be improved and how. The next time you get in the plane work these things and eventually you will build a good cockpit workflow.
As for changing radio frequency, that becomes 2nd nature after a while. If using ATC services (such as flight following, and I highly encourage that), you'll be prompted to change frequency from them to the local field after you let them you know you have the field in sight. If not using ATC, you'll find you know to change when you don't hear anyone from the field you are landing making any broadcasts. And sometimes it is quiet, no one around, but you will still be broadcasting your position as you start lining up to enter the pattern.
For fuel, you can tack that on to various checklists so that every time you do anything, it becomes a habit to check whether you're on the fuller tank. For cross-country trips, the "anything" includes passing each checkpoint--which should be a lot less than 30 minutes apart. There are also apps and other devices that can be set to go off every 30 mins, but IMHO that's a crutch; if it fails, you may not notice and end up running one tank dry.
For radio, you should be on CTAF/Tower within 10nm or so, but if you don't have a GPS/app that tells you that, I'd add a checklist item to switch to 121.5 at Top of Climb (TOC) and to the destination CTAF at Top of Descent (TOD). That's if you're not using Flight Following, of course. Your CFI may not want you using FF because it might become a crutch or distract you from other things you need to focus on at this point, but do make sure you get some instruction and practice before your checkride.
Dave's suggestion to make your own checklists is a good one, especially for things that always happen at the same phase of flight. For example, something that you always do at Top of Climb could go on a personalized checklist. If you do make personalized checklists, just make sure to not omit something from the approved checklists!
That said, I don't know about you (and you don't say anything about where you're located), but one of the first few things I practiced doing even before setting out on my first cross-country navigation exercise with the instructor onboard, was to make a flight plan.
Now, I'm not talking about a full-fledged IFR flight plan complete with exact compass headings, specific wind correction, and so on. This was for daytime VFR flight, so much of that wasn't really needed.
What I did was to sit down at the kitchen table at home with an aviation map of the area and a flight plan form, decided on checkpoints, and figured out how to get from one checkpoint to the next, including determining the distance. For each leg, I'd note things like altitude restrictions (above 2000 ft AMSL here; specific clearance (from whom?) or below 3000 ft AMSL there; ...), a rough compass heading (I was navigating by landmarks, not instruments, but knowing whether to aim for 200° or 160° for a southerly leg was helpful), frequencies and call signs, and so on. Once done, I took a highlighter pen to a copy of the map and marked the planned route; I then carried that copy of the map, the flight plan, and airport directory sheets for both the intended airports as well as all other nearby airports, with me.
Once you have something like that, there is no real reason why you couldn't mark a checkpoint with some sign or text that tells you that you need to do something in particular. You could, for example, write "tank" in the margin to remind yourself that you should switch fuel tanks around that point, or "call XYZ 15nm" to remind yourself to call someone in particular and report your position as being about 15 nautical miles out.
While you should still be able to fly safely without any of this (so you do need some mental picture of, say, where the nearest airport is should your instructor suddenly tell you that "you just lost all your maps, and the radio's shot"), everything that you do on the ground before takeoff is usually one less thing you need to spend significant time and mental energy worrying about while in the air. Choose wisely which things to spend your energy on while you're actually flying.