There are numerous web sites devoted SOLELY to this question!!!!
In short, using FFMs to upgrade when on personal travel as EastMids tells us, has long been established as the most cost effective use of them.YES they are cutting down on the number of reward seats available in each flight but you can shop around.
To gain points/miles I use a credit card that directs points into a particular FFM scheme and so all petrol and supermarket purchases also count.
YES the cost of BAs luvverly 'taxes' can be high but, when I needed to do a LHR/JNB this year, I got WT+ both ways for £370 (all points) and I could not have done that any other way.
If you are going to collect - then collect carefully. Ensure that they all (Hotels, car hire etc.) pile into a single account and then use it for upgrades. If you are NOT going to collect, then shop around. Incidentally, I still shop around every single time because I no longer travel on business as I did before and so have fewer earning opportunities. Some points I have kept ticking over for years until I find the right balance of points/money or plain money.