Not knowing what your conference is about this is a total shot in the dark. Back in the day I did event planning. Did a conference where Jamie Dimon (CEO of JPMorgan) was one the speakers.

1. Start with a teaser site to collect emails and build a following
2. Create social media accounts if you don't already have to them to start engaging your target audience
3. Do small releases of information as time gets closer
4. Press Releases
5. Launch website with featured speakers, sponsors, etc
6. Have your speakers and sponsors cross promote
7. Full court press