(Running Shoes) Additionally, running through the tall grass increases the chance of being attacked by a wild Pokémon.
If a Pokémon with Illuminate is in the first place in the party (even if fainted), the wild Pokémon encounter rate increases by 100%. It shares this trait with Arena Trap and No Guard.
From Pokémon Emerald onwards, if a Pokémon with Arena Trap is in the first place in the party (even if fainted), the wild Pokémon encounter rate of all Pokémon is doubled. It shares this trait with Illuminate and No Guard.
Outside of battle, if a Pokémon with No Guard is in the first place in the party (even if fainted), the wild Pokémon encounter rate increases by 100%. It shares this property with Arena Trap and Illuminate.
Putting a fainted Pokemon with Arena Trap / Illuminate / No Guard with Shinx on the second slot still boosts the encounter rate, which is a good way to get encounters in spite of Intimidate.
Sweet Scent causes wild Pokémon to appear when used as a field move, provided the player is standing in an area where wild Pokémon would normally appear. Sweet Scent will cause a wild encounter even if Repel is in effect.
While not an exact way to increase rate of encounters, it forces an encounter so I think it's worth to mention.
Similarly, Flash Fire boosts Fire-type encounter rate, Static and Lightning Rod for Electric-types, Storm Drain for Water-types and Magnet Pull by Steel-types by 50%. A Pokemon with either of these abilities leading while fainted still boosts the rate.
These increase the encounter rates for specific types only; it's useful when you're EV grinding a certain type of Pokemon.
Source: Bulbapedia (from different articles for each ability)
Hope this helps; let me know if I missed anything :s