It is very important that we concentrate on the internal as well as the external. Nei-dan training should be practiced by all martial artists.
Nei-dan in yin to wei-dan's yang. Nei-dan training can also be broken up into a yin and yang portion. The yin and yang portions always complement each other and make a stronge whole.
The yin side of nei-dan training can take many forms , the most popular being qigong. There are many qigong sets for many different effects. Some are for health, some for martial power, some for energy projection.
What type of nei-dan training were you doing?