Euphoria
The easiest way to double your Score Multiplier for a reasonable little while is the Euphoria button. Euphoria can be used once you've built up your Euphoria bar (located on the left side of the note highway). You earn Euphoria by completing an entire Euphoria section on the note highway (indicated by everything glowing white) without
any mistakes. If you make a single mistake, you've killed your chance to power up your Euphoria with that section... UNLESS you also have a rewind. If you have a rewind available, you can rewind when you mess up and you'll have a second stab at getting some Euphoria, but more on that below.
It can be exciting to earn Euphoria, what with the meter filling up on the left and the button flashing on the controller and such, but just because you have Euphoria available doesn't mean you should activate it immediately. Euphoria does two things for you: it doubles your Score Multiplier (if it's not already being doubled by another effect) and it automatically handles your crossfading for you. Based on this, the best time to use Euphoria is right when you reach a section that you don't want to (or can't successfully) manage a tricky crossfading part. At the very least, don't activate your Euphoria until there are notes that you're about to play. Euphoria is limited by a time duration, not number of notes, so don't start to use it until you're about to play some notes.
Rewind
When the double triangle icon lights up above your score multiplier, that means you have a Rewind available. You can activate this rewind by spinning the turntable platter backwards. Once you've started activating the Rewind, however, you'll need to quickly get the platter back in position so you're ready to continue playing. You also need to adjust the crossfader (if necessary) to match the section you have rewound into. You have about a second to adjust the crossfader so that it's in the correct position before you'll get the dreaded scratched record sound and reset your Score Multiplier. During a section that you have rewound, your Score Multiplier is doubled.
The amount of points you can get from a Rewind is based on how many point opportunities are available in the section you Rewind, the value of your current Score Multiplier and whether you can successfully initiate and execute the Rewind without making a mistake. Rewinds can be tricky, but can be used strategically to squeeze more points out of high point-value sections (such as long scratches) or take a second stab at a missed opportunity to gain Euphoria.
Effects Sweep
There are certain places on the note highway where you will see an orange arc over one or more lines in the highway. After this indicator, the lines are a little bit fatter for some duration. If you tweak the Effects trim knob during these sections, there will be an audible change to the sound (think "Flanger"). In addition, you will earn points. And, of course, your Score Multiplier will double... but
only while you are adjusting the knob. If you adjust the trim knob a little bit and then stop, the Score Multiplier effect and the earned points will stop almost immediately. They will continue again just as quickly if you tweak the knob some more.
The thing to remember here is that the effect is temporary; if you're not going to be able to keep twisting the trim knob, you're not going to continue to gain points or keep your multiplier, so it might not be worth it. Specifically, there are sections that have this Effect, but also have some crossfading that needs to be done. Unless you're making some really fancy moves, the same hand that would do the trim knob is used for crossfading. Now, depending on the upcoming stretch on the note highway, you might be able to continue working the effect trim with your off hand and reach over to crossfade with your scratching hand, but not if there are any notes to hit or scratching to do.
While the Effect trim is a good way to make some points quickly, I find that it's best to use it opportunistically. If you're entering an Effect area and you can trim, work it. If it would raise the complexity much and there's a good chance that if you try to trim that you'll make a mistake and bomb your Score Multiplier, then ignore the trim.