I can't get a trill or tremolo to sound natural.  Are there any techniques for doing this, particularly in Staff View?

Play it?
Also, some instruments are simply better at this than others.

You don't mention what softsynths/virtual instruments you're talking about. Basically, it's the velocity values that are used to create the accents that make them sound natural. Also, the samplers/sample players may have varying amount of velocity layers and the quality of samples varies.

Interesting question.  My Vienna Symphonic Special Edition (and SE+) has actual recorded trills: so if you want a violin to trill, you just hold a note after pressing the appropriate keyswitch.
 
On the other hand, Vienna Dimension Strings—which are otherwise phenomenol!—expect you to create your own trills.  They provide some special samples and expect you to make your own.  I have never gotten this to sound right and even the examples they provide don't sound good to me.  So I end up using the sampled trills from the Vienna Special Edition.

For orchestral trills and tremolos, you are probably not going to get what you want without proper articulations that support this. You can try and fake trills, and if it isn't up front in the mix, it might be be passable. I'd suggest taking a look at some of the better libraries that are available that have the articulations you would need.
 
Regards,
Dan
 

Thanks to all respondents so far.  At this point I am using only hardware sound sources -- a Korg Kronos 88 and a Roland XV-5050.  I can try to play the trills on the Korg and see if I can make it sound reasonably good.  The only soft synths I have came with Sonar X3.

If you have a patch the does not re-trigger—that is, there is a soft attack when you play connected notes instead of a sharp attack when you play separate notes—you can usually get a decent trill by programming notes that are about 1/10 or 1/12 of a quarter note in duration and overlapping them slightly (but it depends on the tempo).
 
For example, if you are using 120 ticks per quarter, you might put notes 10 ticks apart but give them 12 tick durations so they overlap enough to prevent re-triggering.
 
For an ensemble like a string section, you can do this on two tracks and make sure the notes are not perfectly aligned—for example every 10 ticks on one track and every 8 or 12 on another.  Some instruments play the first note of a trill a little longer than the others; like: daaa,di,da,di,da,di....
 
This is tedious but once you have it you can copy it.  Whether or not it sounds good just depends on the patch and the timing.  I have spent 30 minutes making a trill, decided it was too fast or too slow, and then spent another 30 minutes redoing it.  Such is the pain of an artist.

See above for trills, but, for tremolo, what instrument are we talking about?  Piano is pretty simple and wind instruments don't tremolo much, so I guess you mean strings.  Almost every synth with string patches has a tremolo patch but it could be called agitato.
 
In the ancient days before samplers, we did this by using a low frequency oscillator to control the volume envelope, but that was a very poor simulation.
 
Even with sample libraries that have tremolo samples, I usually start with a hard attach sound (like sfz) and crossfade into the tremolo to give the sound a dramatic entrance.

You need the proper articulation sample from the instrument. Anything else sounds fake. You may get away with a trill if the attack is only on the first note. Tremolo, no chance whatsoever.

Hardware synths are notoriously bad at this compared to extensive scripting softsynths like Kontakt or VSE. The Roland Jupiter-80 and its siblings do have some scripting and may offer you slightly better results, but for that kind of money you could also get a kick-ass software library.