You've stumbled upon Proph's self-rule list. Have a look!
1.) NEVER GIVE UP. You can call a map too hard, or call an osu! session a day, but don't resist the fact that you improve with each and every play. All the time.
2.) Being tired is fine. Being sweaty and panting is not, so take a break and drink water occasionally.
3.) Play osu! bidaily. My short-term memory of osu! playing takes around 24 hours to cool off, so every other day I play fresh anew.
4.) Start a new session by playing a challenging map, but one that you can FC within 1-2 tries.
5.) When in doubt for playing maps, avoid favorites and hit F2.
6.) 1-2 retries is ideal. I'd only go past that if I KNOW that I am capable of setting a notable score for sure.
7.) Download at least 1 new beatmap every session. (multis help)
8.) Know your limits, and challenge them. It is not really very rewarding to "challenge your limits" by playing high star maps far beyond your skill.
9.) Play all kinds of maps. Stream maps, jump maps, marathons, control-based pattern maps, etc.
10.) Always, ALWAYS follow the slider to the end. Unless the map is explicitly designed to utilize follow circle ranges, follow all of them.
11.) It's helpful to read the first note of a stream very closely.
12.) Loss prevention is only for really hard parts. Play mostly for accuracy.
13.) Repeater Sliders that are longer than a little bud should be followed.
14.) Remember to properly stream United.
15.) Learn to love all the music as well as all the kinds of mapping (including 2006 mapping)
16.) Holding super fast repeater sliders should be based off of the beat, but when in doubt sightread the next note and only transition when it's time.
17.) Never mash triples or doubles.
18.) All jumps must be made on snap-reposition motions. No smooth sailing when it comes to jumps.
19.) All streams must be made on smooth sailing motions. Only snap the first note if you desire, and hanZeR-esque double-jump streams are not streams but jumps rather.
20.) Mods are situational. Players should mostly work on general, base skills in osu!.
21.) Don't avoid CS5+.
Lastly, as what I'd consider the most important rule...
22.) Find your motivation and keep it. Friends, fun, music. You learn how to be modest with people and music (as well as not infinitely raging on games), while boosting your progression far faster than any trick or any practice would ever do, in any game ever.