On Refinery, as an alien I like to hide on the ceiling in the room that the marines more often than not camp in, the room with the ramp and the catwalks (not the room with the smartgun). You're hidden in the dark and most marines don't know to look all the way up, what with being too focused on the walls and floors. From this vantage point I like to focus pounce on the marines at random.
Now that strategy alone will get you killed. The real trick is that you don't finish the marine that you just pounced off since that would just lock you into melee and have you shot in the back by his friends. Instead what you do is leap straight back up onto the ceiling and reposition yourself for another pounce. The marine you just hit will want to heal, and the others will be looking for you, so make sure you don't attack someone who's blocking. Just rinse and repeat. If marines are camping in that room and you'll rack up that kill streak so long as you're quick on your escape. I've taken on about 5 or so marines at once and beaten them with this method.
Which brings me to my other tip as an alien: run away! Think of the alien as a bullet. If you hit the first time, feel free to mess your target up. However, if you MISS your attack, either leap onto whatever ceiling or wall is closest to you and sprint away. If you're not close to a wall then just skip ahead to sprinting away. It does you and your team no good to increase the other teams' frags. You can always set up another attack run if you mess it up, but only if you're alive.
Oh T.L., it's easy to say run away but the predator will just engage in melee or plasma you on the way out if you miss.
You're an alien. Why does anyone, nevermind just the predator, even see you coming or going? Quit running around in the open, you look stupid. You're supposed to be sneaky, like a ninja. Ninjas don't take their opponents on head on. And if those bullies are catching up to you... well, they shouldn't be. You're the fastest kid on the track team. Quit being silly.
These things keep my pace going when I play as a bug, though my preference is marine. It's so much more satisfying taking down these badass monsters when they can rip me to pieces in a moment.