Do you have your own riffs that you use when watching MST or any other shows? Let’s hear 'em!
A few of mine are:
Whenever someone says something along the lines of “stop him!” I respond with, “Or what?”
Someone relies on an animal companion for help: “Get the C clamp, Lassie!”
Also references to Little Britain: “Yeah, I know” and “Computer says no”.
When someone in a movie or TV show says “Who are you?”, I respond with:
“I’m Batman.”
“I’m a grimwold warrior.”
“I’m the god! I’m the god!”
I went to see Starcrash when it came out because the Topeka Daily Capital’s reviewer gave it the worst review I had ever seen; it is still the worst review I have ever seen. I had to go and experience the show. There were only 6 people at the showing and they almost canceled it; however, they ran the movie and about 5-minutes in one of us said something out loud and we all laughed; from then on, we riffed the movie. Mind, it was years before Mystery Science Theater 3000. I still say some of those things when I see the movie. I’ll not say what they are because they give away plot elements and they resemble many on the official riff. 
Twin Tails is another movie I riff. It is a Russian animated film that has aliens who act as if they are 4-years old, accents that make no sense and are inconsistent, weird music choices (in the English dub), and makes little sense. I say things such as, “These are the ones sent to save the universe,” “OK, Russian accent, Liverpudlian accent, American accent, and all are supposed to live in the same area.” I had my wife laughing so hard until she realized the movie is nuts and just have fun with it.
My personal riffs are usually expletive-ridden and just grumbled quickly into my coat collar so I don’t scare people. That’s why I rely on the show for help so much. 
I have a doggo voice I use whenever there is a dog on the screen. I got it from my ex-wife, and it is pretty funny. I like giving the dog’s internal monologue. Usually it is plot-specific, but sometimes I go for what the actor dog was thinking, which is usually, “damn that treat my trainer is holding looks good,” only funny-ish.
This!!!
Do you do the voices?
There are so many instances of the deadpan flat lines in Jurassic Park, such as “they move in herds… they do move in herds.” I usually add “snakes, why’d it have to be snakes” in a monotone afterwards to reference a certain famous archaeologist and whip enthusiast.
Please tell me you deadpan it like a mo’!
I have riffed with “hold onto your butts.” I think Nedry’s “ah ah ah” computer thing is ripe for use in riffing, as are “sdhoot her,” “clever girl,” and “they should all be destroyed.” Muldoon and other game hunter types in those movies are a great source of material.
Yeah a lot of my personal riffs usually involve using quotes from other things to poke fun at what I’m watching. Jurassic Park has a lot of ones I use, and I also borrow a lot of Futurama zingers
I didn’t know Lash La Rue was an archeologist, too!!
All right, I remember one now, and it’s a little over-the-top but here you go.
Years, before RT went after Original Recipe Star Wars, I saw Return of the Jedi in the theater with my pals. After the final showdown when Luke is reunited with Leia, I couldn’t help mumbling, “Oh, great. Lord Byron’s back. Everyone give a cheer!”
For those who don’t know, Byron was rumored to have had an affair with a half-sister, though scholars are still arguing about that. (Like they do about everything. )
I don’t do much with riffing on my own. I mean, I’ll make comments but they’re not funny or memorable. However, I do have one riff from RT’s Setting up a Room. I find the riffing entertaining but the film is clearly used for a teacher education program and they treat it like a movie and so I make comments on their riffs.
At one point, Bill says, “Now that I think of it, all these things could be undone later and maybe this doesn’t require the planning level of the Normandy Invasion!”
My response: “You’ve never taught kindergarten, have you.”
Riff-back deserves its own thread.
Thanks to the Skydivers (which I’m re-watching right now), anytime I see a small single-engine plane on TV or movies, I say that it’s Petey Plane.
Funny someone mentioned Return of the Jedi.
When the Super Star Destroyer is fatally crippled and crashes into the Death Star (getting destroyed in the process), I like to say:
“Service engine soon? What does that mean?”
“Oh yeah…Vader thought he could squeeze one more trip to Endor of this jalopy.”
I am literally made of MST3K and first ten seasons of the Simpsons quotes, I have no originality.
My friend, however, did the funniest riff 15 years ago that we still laugh at. In the horror movie The Mist, SPOILER there’s a scene towards the middle where the monsters finally break into the supermarket where everyone’s been hiding. I’m not going to link it, because like the rest of the movie, it’s incredibly graphic. But at the end of the attack, as a few main characters have died horribly and they kill the last monster, my friend said ‘Welcome . . . to Jurassic Park!’