The best advise I can give is have a plan of what you want to improve on. Lets say you have half an hour a day to practice. Break it down into five or ten minute sections and work on different aspects say a chord progession that you find difficult. Spend time on strumming patterns with a metronome, picking ect. Pick a song that you really like and break it down into sections. Learn each section really well before moving on, and the best piece of advice is to take it SLOW. You will find the slower you practice the quicker you will improve and become a better player.