Children's learned these lessons at the school of hard knocks:
Before signing on for any large-scale business application integration, make sure the project has non-IT executive ownership--and know the execs' names.
Crash the party: Force your way into meetings at the earliest possible stage; don't let businesspeople minimize IT's involvement.
Pick a code-freeze date well before the go-live date and stick to it no matter what anyone says.
Test, test, test. When it comes to ERP, fight for end-to-end transactional testing.
Know your audience. Spend time plumbing the knowledge depths and skill sets of your integration partners. Don't let them make decisions outside their abilities.
Size matters. Do proper due diligence before selecting and sizing your hardware environment; don't rely on the vendor to do it for you.
Talk to your customers. Engage end users to ensure what's being built meets their needs. Don't let a vendor start any conversation with "It's been our experience ...."
Love your DBA. Success or failure can hinge on how well your databases are tuned.
Don't rely on the main project management team to cover everything. Build plans for every technology area--development, networks, systems, helpdesk.
Don't underestimate the importance of report generation. You may end up tuning live systems just to get them to print a report.
Train everybody, from end users to developers ... the earlier the better.
When dedicating people to a large-scale project, be sure to back-fill to cover their responsibilities.
Be prepared to fight those pesky but inevitable software bugs. Set a maintenance schedule for your development environment.