I have already made all these changes for my ultra-long lines (ADSL1). Some of these items will not necessarily help _you_ as you may have no problem to fix or a much stronger signal, but they won't hurt.
the list, in no particular order
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* Ferrites on dc power cables (might do nothing at all)
* RJ-11 cable, best quality, v short - Tandy / adslnation
* Mains cables away from other cables
* Mains filtering and conditioning (esp. if using a UPS)
* Good faceplate microfilter aka SSFP, no danglies
* Correct faceplate filter choice for service type, maybe avoid BT Mk3
* No extensions, or no phones
* If you absolutely must have extensions, make sure they are prefiltered in the faceplate
* Don't put the modem on an extension! Rather, move the router if a separate unit or better still move the NTE5
* Modem change?
* NTE5 placement, and length of internal BT Openreach wiring