The basic rules of thumb to remember are:
1. If you double the number of creatures, add +2 to the EL (so 2 creatures that are CR 4 become EL 6; four of them become EL 8; etc.)
1A. As a corollary to this, if you increase the number of creatures but don't double it, add +1 to the EL (so, continuing the example above, 3 creatures that are CR 4 are EL 7, and five to seven of them will be EL 9).
2. In a mixed encounter, a creature of CR x and a creature of CR x-2 are EL x+1. So, for instance, two creatures are encountered together; they are CR 7 and 9. The encounter's EL is 10.
Applying these two rules of thumbs will allow you to estimate the EL of most encounters, but this can break down pretty easily above about a dozen creatures.
Example 1: You have a group of four CR 3 gnoll warriors led by an ogre shaman who is CR 8. The overall EL is 9 or 10 (four CR 3s = EL 7; CR 7 + CR 8 = EL 9 to 10).
Example 2: You have a wildly diverse group of creatures, including 45 CR 1/3 goblin warriors, three CR 5 goblin leaders, four bugbear bad guys at CRs 6, 7 and 8, and a nasty rakshasa sorcerer who is CR 15. What's the EL?
You have to break it down, treating each EL as a single creature, until you get to the end. So:
45 CR 1/3s = 15 CR 1s.
3 CR 5s = EL 8.
Now we have:
15 CR 1 (= CR 8)
One CR 6
One CR 7
Two CR 8 (= CR 10)
One CR 15.
That leaves us with:
One CR 6
One CR 7
One CR 8
One CR 10
One CR 15.
So we add again. The way I would figure at this point is 'bottom up', so:
CR 6 + 8 = CR 9.
CR 7 + 9 (generated by the 6 + 8) = CR 10.
That leaves:
Two CR 10s
One CR 15.
The two CR 10s = CR 12.
Now's the interesting bit. A CR 12 + CR 15 =
EL 15... so all that figuring was wasted!

Now, you might think this is bunk, but think about it- will a collection of 45 goblins really be a challenge to any group of 15th level adventurers? Not likely- one
fireball and they're all dead. They're just a chance to show off for the guy with Great Cleave or area of effect spells. The rest are almost enough to toughen the EL as a group, but even the CR 8 bugbear isn't really gonna last more than a round or two in a battle with 15th-level characters. Although it
looks like a tough battle, it's only the rakshasa who will actually do much against the pcs.

Hope this helps, long convoluted example and all...
