Mantra work and Self-Remembering.

If you've done basic mantra work for a while, and have built a decent foundation for "access concentration" (or mindfulness on an object), the next step is to add some Insight practice.

In my experience, I've found mantra work plus the later step of "self-remembering" (or maintaining a steady awareness of one's self doing the mantra work) to be extremely potent.

It also has the benefit of making it virtually impossible to "fall asleep at the wheel", or get lost in imagination, while performing meditative /spiritual practice.