To start things off, I have something I didn't quite understand. While I was grinding my boomburst taillow to a high level, I killed over 100 zigzagoons. Then I wondered what I could get from them if I used the pokefinder. When I found a zigzagoon, like 4 tries, its hold item was a revive, when I saw this, I had to switch to my grovyle and false swipe it (can obtain false swipe TM from the guy standing near the counter of the pokemart in rustboro) the zigzagoon, and caught it with a poke ball. It made me think, so I kept searching, and about 10 of the first 20 I found had revives, 6 had potions, 4 had nothing (including 2 3 star potentials). The reason this is good is because if you catch the Pokémon with a revive in either a great ball or a pokeball (pokeballs are guaranteed, great balls are a waste here), you actually make a profit (revives sell for 750 poke).

side note- if you want to get up to 100 zigzagoon encounters, you just need a Pokémon with sweet scent (lotads get that sometimes if you use the pokefinder to get them) to summon hordes, and use a hit all move, such as razor leaf or boomburst (taillow is the earliest I've seen for this, same thing as lotad), I'd recommend boomburst, as its got 100 accuracy and 140 power, 10 uses,only drawback is it's move-type (normal)

if anyone else has a better method, please post it here so the community can learn

So this may not be the best since it's been a while since I played through a Pokemon game and Emerald is the one I'm the least experienced with. If you want to be very efficient I'd say get the amulet coin which can be gotten after defeating your dad I believe and going back and talking to your mom (doubles the amount of money you earn in a battle as long as the Pokemon is used in battle), choose a Pokemon power level it and let it hold it. Make all of the other Pokemon have pickup so you get free items and keep battling the elite 4 (this would be assuming the Pokemon is around level 100.

If you haven't gotten to that point, I'd just do something similar but with regular trainer battles and using the match call feature which let you rebattle others.Having all Pokemon know the ability Pickup is optional so if you feel the game would get boring by only using one Pokemon for battling you don't have to.

I have felt the fastest way to grind Poké in most generations is to beat the Elite Four with an Amulet Coin or Luck Incense.  However, there may be better ways to grind for Poké in certain gens.  There's a rich man's house (I forget his name) in Gen V and he has a family that pays out big time when you beat them (if I remember right, you can only challenge them once a day).  There's also a limited number of stuff to be found in an underwater area that can be sold to this man for big cash.  In X and Y, you can work at the hotel for an increasing payout as you "rank up".  It may not pay as quickly as beating the Elite Four/Champion, but it sure can be fun.  In Ruby/Sapphire/Emerald, there are a number of trainers that pay well when you beat them (most notably Rich Boy Winston and Lady Cindy).  These battles can be quickly finished with decent level Pokémon.  I hope this helps!

I remembered doing a classic method on Pokemon Pearl when I was about 8 to be able to grind infinite (999,999) Poke, haha.

How it worked is that I had a Pokemon at the top of the party equipped with an Amulet or whatever it was (If I remembered correctly from years ago, it doubled the Poke you earned from winning battles.) and I use that battle charge item (Again, haven't played in forever so don't remember the names.) Which after enough steps, I go to an area with a bunch of random trainers on one of the bike roads, and use the charge item to make everyone ready to fight again.

So then I fight all the trainers, win the gold, then after walking enough steps, I fully charge the battle item and use it again, rise and repeat. Just remember to save, of course. Good times.

I loved messing around with Zigzagoon. I would catch like 5 of them and put them behind my starter and then I would just keep running back and forth, battling my starter. Not only would this be regular level grinding, so my starter would get like an extra 5 to 10 levels, but all the zigs got me so many items, even a few rare candies which I was able to use right away to OP my starter as well as a few other epic items.