What i normally do is , once the song has been written i look at the key in wich i am soloing over and decide which scale mode(s) to use, then i 'jam' a solo over the song a few times and start to build up ides for a solo, i would pick out certain licks/runs that work well and kinda fill in the gaps with ideas that fit and sound right for the song .Quite often i will hear in my head what i want and then its a case of getting on the fretboard..Hope this helps.