Some tips:
1. Try to learn the level first, fail a couple of times but learn the level layout, where all the tags are.
2. When tagging, go for big tags first. In about every level, you are chased by someone or something, and if you have big tags out of the way, it'll be a lot easier for you.
3. Combo moves together for more points.
4. Spray green tags for bonus points, these are not required but increase your score.
5. Spraying x amount of red tags makes the cops and the woodwork come out, keep this in mind and remember how many tags you've sprayed at any given time. There will always be a warning on the cop feed for when they're going to go all out. Green tags are good for this in later levels like Fight or Flight, because you may find yourself having to high tail it for red tags when you're being chased by dudes who are shooting the ever living fuck out of you.
6. Each character has their benefits. Gum is faster than Beat, but Beat has more health. Gum can hold 15 spray cans whereas Beat can hold 20 cans. Gum can less cans but her sprays are more elaborate compared to Beat's more simple tag motions, meaning more points. This means that Gum will net you more points than Beat and she's harder to play. The stats to keep in mind are power (health), technique (speed/tricks), and graffiti (graffiti can count/more elaborate graffiti). Try each character to find their subtleties and your own preferences. Personally, my favorite character is Pirahna, because she uses elaborate tag motions which result in extremely high points if used in combo, but some people may prefer Yo-Yo, who uses simple tags, but is fast, and can score you big points in his own unique way. So if you're struggling on a stage, try it with another character to fit the stage.
7. Try to use everything to platform, escape and maneuver on.
8. Use boost only when trying to get speed. Think of it like Sonic the Hedgehog. JSRF has no momentum or speed to account for, JSR does. It's like Sonic 1 compared to Mario 1. Mario's sense of gravity and momentum is non-existent, just like in JSRF's; but in Sonic, you need to go back uphill and gain some speed in order to go through that loop. JSR works the exact same way, try to notice when and why I use boost in the vid I posted, this is crucial. Always boost before railing.
9. Enjoy. Have fun!