The basic premise is that Riolu can only evolve into Lucario with maximum happiness between 04:00 and 20:00.
How you reach maximum happiness with Riolu is down to you as a Pokemon trainer, as each specimen can be different.
Maximum happiness is usually attained at 220, and there are a number of things you can do to help boost your Riolu's happiness:
- Walking him plenty (250+ steps usually does the trick)
- Massage (either in Robbon Syndicate or also in Veilstone City)
- Giving him vitamins
- Trying a level up
- Make use of the EV Berries
A word of warning though!
Make sure you defeintely ready to evolve your Riolu, as there are several really neat moves that only Riolu can learn, so you may want to train him up nice and good before making the decision to evolve him.