This is a very broad question. There is a whole industry around this. A good resource for this type of information is Continuity Insights. There are questions you need to answer like:

1. What is your recovery time objective? How quick do you have to have some "system" services available?
2. What is your recovery point objective? Should you be able to recover up to the last 15 minutes, last hour, last 24 hours, last 7 days?
3. What critical services need to be recovered?

The network is just a small part of this type of activity and the answers to the above questions will help you design the network with your DR objectives in mind.

From Carrie Higbie
You say that you have failover installed (first I recommend testing it). But you need to determine the critical apps that need to be mirrored. Are you mirroring storage to your DR site? If not, what is your plan to restore data? You also need to determine what "acceptable" periods of downtime are for your company. Ideally you want to make sure that you have the exact same equipment at both sites. Another VERY important step is to be sure that you log everything at your main site. This is one thing that most people miss, but when you restore it is critical that you have the same patches and code loaded to assure that your new site will perform the same way.

The close you can keep the structures the easier things will be in a disaster. Bear in mind that you will want to make a business decision as to what applications and services are needed. Make sure that you maintain a current list of contact information for the disaster recovery team. You can visit www.Siemon.com and your local sales rep can get you a copy of the business continuity and disaster recovery plan.