Best practices for coding in Exits

Does anyone have any documents on the Dos and Don'ts in BADIs, Exits and SD routines from the angle of System performance and how to minimise the load. Iam not talking about the generic Optimisation documents but something specific to SD routines and general BADIs and Exits which fall under the Best practices (coding) of SAP. eg. One guideline from SAP is that Clients should not tamper with the 'x' variables (xvbak, xvbap etc.) in the SD form exits.

Thanks,

Madhavan