Do you want to write Orchestral String Ostinatos that Sound Epic for your Cinematic Music Compositions? 
Let’s share tips on ostinato writing, layering, sound design and all aspects of this essential technique for energy and drive in modern cinematic music. I will start by giving my tips, and I hope to see yours in the discussion of this thread.
1 - Rhythmic, Harmonic or Melodic?
First I decide if the overall groove of the ostinato with all parts, should be rhythmic (as on monophonic drive focusing mainly on one pitch), harmonic (similar to an arpeggio by following the chord notes more), or melodic (providing a melodic focus inside the ostinato like the Pirates of the Caribbean theme).
2 - Start with the Main Accents
Then I start with the main accents of the ostinato. Meaning the fundamental core groove which the listener will hook on to. I often start by playing this in octaves in the lower range (Basses and Cellos, or an ensemble strings patch). I also make sure to make them higher in velocity so that they really are accented in the overall groove when I add all other elements.
3 - Recording Tricks
Personally I most often record in all parts in my music, even ostinatos. Since faster rhythmic parts is quite difficult to record in time, I have some tricks for it. The first is to lower the BPM of the track just for the recording stage. The second trick is to record the part in half time, then time stretch to make it double speed. And the final trick for recording, is to use “step input recording” which is a feature in most DAWs where you record the notes step by step without having to worry about timing.
4 - Layering for Clarity & Power
I love layering in music composition. For ostinatos I have two main ways of using layering. The first is to layer for clarity, where I might have the main ostinato line played by a lush string sounds with lots of room sound in it. Then I layer a very dry string sound on top of it for that crisp clarity. The other way I love using layering is to simply add depth and power. Especially for the lower range and main accents. For example, I might have a double basses patch playing the main bass ostinato, with a cello an octave above playing the same line. Then I sometime add a string ensemble playing octaves, following what the bass/cello combination is playing, but only on the main accents for extra power.
5 - Variation in Dynamics & Articulations
Variation and movement is essential in music, and especially dynamic variation (velocity values in MIDI). But I also recommend adding variation in the articulations the ostinatos are playing. From the shortest spiccato, to staccatissimo, to staccato, all the way to marcato for more pronounced accents.
PS. I also have a course where you can learn to how to master ostinatos, check it out here: Master Rhythmic Ostinatos in Cinematic Music.
Now please share your own tips on Ostinatos in Cinematic Music! 
Sincerely,
Mike