The following is taken from my response to Glenn Fleishman’s Macworld article about getting Back to My Mac working on your Mac. FarFinder and Webjimbo users can encounter similar issues, so I include this here for anyone interested. It will make more sense in the context of the original article.
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The issues Glenn describes above are common to any remote service you run on your Mac, including FarFinder, so I thought I’d contribute some of my own experience.
The single most common problem is when people with DSL connections buy a separate wireless router. This causes the “double NAT” problem Glenn describes – you now have two private networks: one between the modem and the router, and one on the other “side” of the router where your Macs are. Rather than putting the router (or Airport) in bridge mode, I usually recommend telling the modem to forward all incoming traffic to the router (“DMZ” or “default server” setting). This is because most routers support UPnP, while many modems don’t, so the poor user is often saved an extra configuration step.
Alternatively, the simplest solution to the double NAT problem is to buy a decent DSL modem with a built-in wireless router, thus you only need one device.
(Incidentally, cable modems are transparent to the network, so a wireless router added to a cable connection doesn’t suffer the double NAT problem, since it’s on the internet-proper.)
Another big problem is that a lot of routers just don’t do UPnP (automatic port mapping) properly, even though they claim to support it. (I go into this in detail in an earlier comment.) Of the big names, D-Link are particularly guilty of this in my experience, and I often end up advising users to add the port mapping to the device manually instead – a hassle, but a stable solution in the long run.
Having said all this, it’s almost always possible to get things working properly on a home network with the right configuration changes. (Exceptions are cases where the user doesn’t have access to the modem and router, eg. some apartments, some offices, schools and most hotels. The ISP restrictions Glenn describes are less common in my experience.)
But the network device makers are letting us down. Settings are hard to find, badly named and terminology varies wildly from device to device, often to the point of nonsensicality. Reliable, standards-compliant devices would be a great step forward, but there’s also a lot that could be done in terms of automatically adapting to common network conditions, eg. the double NAT problem.
Unfortunately the obvious place to lay the blame is on the software – FarFinder, BtMM or whatever, and you get many people mistakenly saying that a product doesn’t work. You can’t blame them – they shouldn’t really have to care about this network stuff – but they end up missing out on useful software because of it and mislead others too. The fact is that quite often configuration will be required and there’s simply no way for the software to do it for you. The best it can do, like FarFinder, is recognise common situations and point you towards some help.
(Of course BtMM might have other reliability issues too – I don’t know, I don’t need it 
One day, IPv6 might solve this all for us, but don’t hold your breath.