There really isn't much you can do make your Shiny Rate higher or faster in Pokemon Emerald. But here are some methods :
Chain Fishing
Get a Pokemon with Suction Cups in front of your party (doesn't matter of its fainted)
And keep fishing. The chain breaks if you move or don't reel up a Pokemon. You can run away/kill/ catch the Pokemon and it will not break your chain. If you catch a shiny your chain resets
Encounter Pokemon
If you keep encountering Pokemon you might be lucky and hitch a shiny
Breed
Masauda Method is not present in Gen 3 . Just get eggs....and more eggs...and lots of luck
Good Luck :D