Informal:
* For life: "We're friends for life!"
* Always: "I'll always love you."
* Eternally: A bit more dramatic, but still informal.
* Till the end: "We'll be together till the end."
* Till death do us part: Often used in wedding vows, but can also be used more casually.
More Poetic:
* For all time: A bit more romantic and evocative.
* Through eternity: A very strong, emphatic way to say "forever."
* Forever and a day: A slightly humorous way to say "forever."
* Until the stars fall from the sky: A metaphorical way to say "forever."
* Until the end of time: Similar to "until the stars fall from the sky," but perhaps more literal.
The best slang term to use will depend on the context and the tone you want to convey.