First, don't leave your paints in the sun.
This year I have set myself a goal of getting x number of minis done in the year. I also set monthly targets for myself. There is a monthly painting challenge on over on the Warlord forums that I have been taking part in. I haven't missed a deadline yet, but sometimes I have posted pictures of my finished minis on the evening of the last day of the month.
I also try to look at painting as something that I want to do, (it usually isn't, I'd rather be on the web or playing games). Then, if I can catch 30 minutes before work or an hour at lunchtime I feel like my time is well spent and don't feel bad about playing on the PC after work.
I don't try and paint models in one go. I batch paint (in 20s, but 4-5 is probaly easier to handle). I break down the painting onto stages, base colours, washes, highlights, skin tones and faces, basing. And then break these down into further sub stages which I use as my target for today.
E.g. Today I want to paint in all the metallic base colours, such as spear points and sword blades. Tomorrow, I'll paint the breast plates, etc. Small, achievable goals work for me.
Do a substage or two a day, do a stage or two a week, do 'y' minis per month, do 'x' minis per year.
I should point out that this is the method I have found works for me. For example, most people paint in terms of minis, unless they have an upcoming event. I paint in bulk all the time. Some minis look gorgeous up close but mine look rough around the edges. I have a massive backlog of painting to work through and I want to get it cleared. Feel free to try out the above, but if you don't like it, don't worry. As Lovejoy points out above, painting characterful minis you have an interest in can be a lot of fun.
Painted Minis in 2014: 510, in 2015: 300, in 2016 :369, in 2019: 417, in 2020: 450