I didn't start thinking in Spanish until I was DELE B1/B2. I put all my computer settings in Spanish, so Excel, Word, and internet explorers are all Spanish. So is my cell phone. I practice every day through readings, videos, and conversations. One thing that threw me over the top was listening to Spanish radio. I listen to a station in Madrid and just keep my earbuds in while I work. I learn slang, expressions, vocab.
The other day I was talking in English and couldn't think of the word Fiancee, only the Spanish form which is Novia. That's when I knew I was thinking without effort from time to time in Spanish.
So read, write, listen, talk in Spanish each day and you will get there.
Comment - numbers are the hardest to think in Spanish. I work with numbers all day and cant just look at $125,554.34 and think to myself cien veinte cinco etc... Just can't do that. But ordinary thoughts in Spanish are easy now. Well easier, I should say.