I remember, not too long ago I made a decision to get 99 defence on Runescape. I still don't have it, but I do know what you can do too keep up the hard, momentous work. I first found out what skill i wanted to train after i asked my friends which were common, easy, fun, ETC.,which ones would make me more money, and what ones would help me in the long run. I chose defence because it was one of my highest stats, has a cool cape, and can help me do things I like to do (tank Bandos, go to corp. beast, KBD, ETC.) I have been playing Runescape for almost 11 years now (and still not one 99 yet!) and just haven't been very dedicated to my 99 until recently. My name is ONLYUSEmeZGS (for now...I'll tell if I change it) so feel free to add me if you want. I know a lot about Runescape since I've been playing basically since the start. I've done a lot of stuff in Runescape, I've wasted a lot of my time trying to get rich (still not there), and I've seen a lot of things. So I'm going to tell you how you can stay interested in that special skill you want to master. Here is a link to all of the possible skills: runescape.wikia.com/wiki/Skills
First thing's first, find out WHY the skill interests you. For example, I like defence because it is used to tank different bosses like Bandos. Keep this interest in mind, try to set a major goal (like being able to tank) and try to attempt the goal until you either get good at it or if you can actually do it.
Many players prefer to take breaks every now and again. I enjoy taking breaks after every other level or so. I also like to find people that also enjoy the same skill and want to train also. I like to go to the place of training and having people to talk to while I'm training. Try new things while training...like trying different looks or seeing what kind of armor suits your attack style.
Depending on what you are training and what you are training on you may be able to afk (away from keyboard) while your character does his/her job. Many people going for 99 do this. The most common skills these are used on are boring, time-consuming skills that have a long animation(fishing,woodcutting,ETC.). If you feel like you are not feeling the same as you felt about the skill before, this could be something for you to look into.
Never put runescape before you real life. Your friends and parents and loved ones do not care much to see you play a game rather than going out and playing a nice game of football or soccer. This can be very dangerous to your grades as well as your future. If this begins to happen take a break from gaming all together. Go out with friends, maybe watch a movie, wreak havoc throughout the city, just go out and have fun.
Find out what you like to train on most. Then you will be less bored when you start do work on the skill for a long time. Along with that find out what experience you like the most and feel you are getting the most of. Don't forget about your 99 party! Everybody has to have one. All that work for no reward? Get real, you deserve a party, so go have one.
I hope these little tips helped you out! Congrats, keep up the hard work, good luck, and everything else you say when someone is working on getting 99. Thanks for reading, I hope you enjoyed.