Apart from the 'health' of your phone line you need to know about:
 

BRAS Profile (IP Profile)

BT Wholesale give each line a speed profile depending on the line's speed and stability. So even is you are lucky to connect at the maximum 8128kbps the most you can actually 'use' is 7150kbps!

IP Profile figures in table below
See table below to see what profile you can expect depending on your connection speed.

Your BRAS profile (also known as IP Profile) is often the reason people 'complain' about a sudden slow-down in their connection! Or also wonder why normal speeds haven't returned after a problem.

You can find out your current IP Profile by visiting this BT Wholesale page:
www.speedtest.btwholesale.com/PerformanceTesterWS/diagnostics.do

If you have a 'low sync event' - which means for whatever reason you connect lower than normal (Bad weather, Damaged line, Router on the blink, BT up your pole!) then your BRAS profile will be reduced accordingly. Now if you manage to re-connect at a higher or normal rate, your BRAS profile will take between 3 hours and 5 days (it's normally 3 days) to return to normal!

So if something has caused you to have a low connection, you should check that your connection speed (Sync) is back to normal and then all you can do is wait for around 3 days for your BRAS profile to increase and give you your normal speed back. If you can't manage to get your normal sync back there may be a line problem - see below.

Yet another thing to be aware of is it's possible your local exchange or ISP is suffering from congestion - so even if everything looks fine but using any of the many speed tests available shows poor results, there may not be a problem. When Freeola's off-peak period (10pm) starts it would be normal to see some reduction in speed as people start scheduled downloads etc. So the thing to do is try a speed test at another time of day to see if you still have a problem.

Test Socket (BT Master Socket NTE5)            More on BT Master Socket
If you report a problem to Freeola (you can only report ADSL problems to your ISP even though BT Wholesale maintain the main network) you will invariably be asked to connect to the Test Socket in the BT Master Socket (NTE5).

If you have a modern BT Master Socket it will have a removable face plate on the bottom half. You can remove this plate which will reveal the Test Socket behind. As you remove the plate it will disconnect all your internal phone extensions - this is so that you can see if your problem is due to something in your house (phone gone potty, faulty micro filter, cat chewed a phone cable!).

Plugging your router/modem directly in to this means you can test your broadband connection, proving that any phone line problems are BT's and not yours.

But remember - even if you connect to the Test Socket and see your normal Sync, your actual speed won't improve until your BRAS profile increases (around 3 days).

Some other 'Rules':

SNR           More on resolving SNR problems here
If your line becomes unstable, the BT kit (DLM Dynamic Line Management) can increase your 'Target SNR' (in 3db jumps) to 9/12/15db.

Each time the Target SNR increases your Sync speed will fall (by roughly 500-750kbps) in an attempt to stabilise your connection. Getting your Target SNR to reduce if it's over 6db should happen automatically after 14 days of stable connection and depending on the amount of errors that are being seen on your line. But in reality this is sometimes hard to achieve!

If your SNR falls too low then you will lose your connection and when the router reconnects the sync speed will have reduced in an attempt to stabilise the line.

Blip logic
If you have managed to have a stable line for 14 days and then have a low connection, but manage to reconnect at your normal speed almost straight away - then within an hour or two, your BRAS Profile should return to normal rather than the normal 3 days'ish.
 
***Unfortunately it seems BT have stopped using this rule***
So if you have a low sync/connection it can take 'up to' 5 days to get your normal profile back!

NOTE: If you are on an ADSL2+ line then the IP Profile will recover much faster than an ADSL one.

Re-connecting/disconnecting your router
As you are hopefully seeing by now - there are quite a lot of factors affecting the state of your ADSL connection!

You should be aware that re-booting your router or disconnecting and reconnecting the phone line, or working on any of your phone extensions etc. can all 'upset' the BT kit which could in turn cause you to get a lower IP Profile! So don't do any of the above too often.

Ring Wire tweak (Improving your broadband connection and increasing your line speed)
If you find that connecting directly into your Test Socket improves your connection speed - as many people do, then this shows that your internal wiring is affecting things. I'm assuming all your phones etc. are already correctly filtered by this stage. If you find your connection speed is better in the test socket then removing your 'ring wire' or 'bell wire' (connection 3) could be worth doing. The theory being that the ring wire is acting like an aerial picking up RF (radio frequency) 'noise' which will affect your connection.

All you need are wires to terminals 2 and 5 connected in your phone sockets - all the other wires are not required and could be introducing interference/noise on your line.

Removing the ring wire in all your sockets won't stop modern phones from ringing as with adsl the micro filter provides this functionality. NB. You aren't legally allowed to mess with anything on BT's side of the Master Socket. i.e. You can remove the face plate and change that wiring, but you can't change the wiring within the main Master Socket where the outside wires come in. If in doubt don't do it!

Some people have seen their connection speed improve by 2 or 3MBits/s - so it's worth looking at!

Micro Filters
When your phone line has been 'ADSL enabled' then you have to have micro-filters on all your phones/fax/alarm/sky phone connections. The filter 'splits' voice and data using different frequencies so that they can share the same line.
Another option to improve your connection is to fit a 'filtered face plate' - this replaces the normal BT face plate (which can be removed) and means all your internal phone wiring is filtered at source - so you won't need any other filters.

BT BRAS/ IP Profile table

Line Sync / IP Profile example:
If you normally connect around 3,200kbps.
This will give you an IP Profile of 2,500.
So 2,500kbps (less overheads) will be the best download speed you can expect.

Low SNR and Router choice
Your choice of Router can make a big difference to your broadband's performance.
First a Netgear DG834 then a 2Wire 2700 and finally success with a Speedtouch 585

Quick and simple speed checker here
View your current broadband download speed

BT iPlate
Broadband Accelerator

ADSL Faceplate Filter