I would first refer to a comment I made previously - implementing an ERP system is a business project not an IT project. It needs to be lead by a business project team, not an IT project team.
An implementation partner can be very valuable if they are the right one - but too many claim expertise in areas that they don't actually know that much about. On top of that, they often claim that they have highly qualified and experienced staff, but then supply people with at best limited experience. (However, they will still charge you the same rate).
You need to make sure that the integration partners are good at knowledge transfer - too many sit in offices without discussimg what they are doing or why, so once you are on your own, you don't have the necessary skills to fix issues in house.
Most big integrators have their own methodology for these projects - in many cases, just having a general Project Manager without experience of the specific methods is a bad idea, as it can lead to serious confusion between people. Make sure that the methodology used is the one supported by the software supplier. Make sure that they stick to it as well - too often, they say they will use the approved process, then end up making it up as they go along.
Document EVERTHING - meetings, phone calls, discussions, emails, and especially anything involving decisons. Too often, having made a decision, something will change - you need to know why something was decided, and after 2 years, you probably won't.
Have a process of reporting issues and make sure that they deal with these. Try to get an agreement on a time frame at least. I know of cases where issues remained unresolved for several years.
Even if they are the very best system integrator with masses of satisfied customers, don't assume that they know everything. The very best people will admit that they often come across issues that they don't understand and need to refer to a wider community for assistance. Sometimes, they will make recommendations that turn out to be wrong - it is a pain, but that is life; accept it and move on.
Make sure that you have someone internally that has the authority to take them to task for failure, and that person needs to be working as an internal project manager. The person needs to be a really good communicator, organised, disciplined and preferably with a really good knowledge of interal processes. This person is worth every penny they earn if they are good.
I would also suggest that good communication is a key element in a succesful implementation - too often, staff are completely in the dark about what is going on. this leads to serious morale issues, and this can determine if your project succeeeds or fails.