What is a quick/fast way for defining activity relationships in complex project???

there are several techniques to do what you want to do.

opting #1

One way is building intelligence into the activity id.

For example you could have the chain of activities for steel for the 2nd floor as 

sub2stl2000 which would be for the submittal of of the steel shop drawings

then app2stl2000 which would be for the approval of the steel shop drawings

then proc2stl2000 which would be for the procurement of the steel shop drawings

then you could have inst2stl2000 which would be the installation of the steel for the 2nd floor

then you could filter on where activity id contains 2stl this would give you the chain of the 4 activities

you could then select all then right click and then select link activities

or some variation of this it is up to you but you get the idea

option #2

another way to do it is to use an import spreadsheet that is available where you create the activity and list its predecessor id then import the activity with the relationships via excel.

here is a link to this spreadsheet see my posting the excel file is here 

http://pmsite.com/forums/viewthread/3315/option #3

option #3

another way to do it is to create one activity on the schedule then export the sanctities and the relationships to excel then use this as a template for importing activities into p6. There will be 2 tabs one for activities and one for relationships the activity tab is labeled as TASK and the relationship tab is labeled as TASKPRED you can populate both of these tabs in the spreadsheet and then import the data into p6.

Remember that any column that has an asterisk in the top of the column can not be imported.

Also when you perform your import into P6 first import the activities then import the relationships 2 step process.