A professional arranger might cringe at the thought of using frequency attenuation on specific instruments to bring out the 'beauty' of the part for the listener.   We know this happens at the mixing stage, but when writing a part, especially orchestral pieces, one should keep the frequency problems in mind while arranging the instruments and parts with each other.
 
My suggestion is to start with the orchestration, and do everything you can there, before moving on to making it sound better in the mix.  Assuming you want the Cello's and Violins in the forefront, make sure the background material is not stepping on the instruments rhythmically and frequency wise, to distract from what you want to hear.  Also, use dynamics to your advantage.  Obviously, if you want to hear the solo flute over the rest of the orchestra, they can't be playing FFF behind it! lol.
 
The ear will natural pick up on a line that is separate in frequency and separate in rhythm than the background material. 

Automation. Also the orchestration itself must be considered.

I definitely agree with the advice here of getting other stuff out of the way.  One note: I really try to avoid EQ on the orchestral instruments (assuming they come from a good, well-sampled library like VSL, EastWest, etc.) and I get a bit bolder with EQ on the less organic things that are in the way (like Rhodes, electric guitar, etc.).
 
This last thing probably doesn't apply to you, but don't be afraid to drop something out of the mix.  In one mix I just finished, I realized I couldn't get the Rhodes loud enough in the busy mix.  After a minute of thought, I realized it wasn't contributing anything and was just duplicating the electric rhythm guitar part—and taking some of the character away from the guitar.  The mute button is now on that track.
 
 

cksh.simon
I use EWQLSO and I did some eq work on cello and cello arpegio sounds good and quite distinct by itself.
When it's played along with high violin and the rest of orchestra, it loses its distinct sound and sounds more like background effect.
I need both cello and violins to be heard distinctly.
 
I'm wondering if the right delay/compression/distortion could help, but I'm not a mixing expert.
Can anyone share some insight?

The first sentence of your post indicates your problem. You have possibly scooped your cello. This sounds better by itself, but it will get lost in the mix.
There are several excellent posts on this thread. They are all correct.
But in your case, all you may need to do is go back to square one with your cello sound, EQ it only while playing it in a mono mix, and see what you get.
 
Everybody in bands that I record want to do "the works" on their track while soloing it. Dynamics processing, EQ, reverb, delay, phasing, flanging... Of course it sounds bigger by itself, but the mix sounds like an LSD Carnival by the lake.
 
It's fine to EQ while soloing to get the feel of your sound, but don't use that setting for your mix.
Try my suggestion, then use some or all of the other tips the generous people of this forum gave. With automation, a lot of this stuff goes very quickly when you get used to it.
 
Now you can slowly bring in a little of the EQ you wanted, with the stage position, timing, reverb, etc. giving you a little more wriggle room to go for that sound you heard in your head in the first place.

Anderton
The concept of cutting frequencies that "step on" other instruments is often overlooked but has wide applicability. When EQing guitar in a singer/songwriter situation, the first thing I do is drop the guitar in the vocal range. The voice becomes way more prominent without having to do anything to the voice itself.
I think the emphasis that both Beepster and Razorwit placed on EQ indicate that it's the first thing to try.

This is such a common, yet tedious, thing that there ought to be better tools to do this.  Conceptually I understand the idea of clearing out space, but in practice this take a lot of time and patience.  I look upon  this as being very similar to side-chaining compressions, except that affects only volume, and not a particular part of the spectrum.
 
Somebody ought to invent a tool that would essentially side-chain frequency cuts.  Imagine if I could send the cello to a side chain, and then run it into a fancy new equalizer on the bus that has the rest of the orchestra.  This hypothetical equalizer would have the job of automatically attenuating the strongest frequencies observed on the side chain, while leaving the rest of the frequencies alone.
 
Is there such a tool?  It seems to me this would be a great aid to anybody dealing with this kind of mixing challenge.
 

I find this a handy plugin for sounds being masked in a mix:

As long as you don't overdo it, or use it just to be able to hear what frequencies are fighting for space and then remove it. It's really easy to use as well.

Sorry, that last post didn't work:
 
The link to the Wavesfactory Trackspacer 2 plugin is:
 
 
 
 

Link not showing up?
 
Plugin is Wavesfactory Trackspacer2 .... just google it.

Haha both were very helpful replies and I thought I could pick more than one reply as the answer.
turns out I can't. so i gave it to the first helpful reply sorry:) 

Jean
I find this a handy plugin for sounds being masked in a mix:
 
As long as you don't overdo it, or use it just to be able to hear what frequencies are fighting for space and then remove it. It's really easy to use as well.

I don't think you can post a link until you hit a magic post count.  Here it is.
http://www.wavesfactory.com/trackspacer.php
 
That looks EXACTLY like what I was describing.  This seems like the easy way to open up space.