What I always do is play the low Bb on the E string with my little finger if the music allows. Also the Eb on the 6th fret of the A string and Ab on the 6th fret of the D string. This will help you expand your playing. Also, I hardly ever use an open string, use the fifth fret instead. Thus allows you to go to notes like 2nd F#, B, C easier.
If the key is D or any key with two or more sharps it's easier to play in second position (as it's called), meaning around the fifth fret. That way C# and F# are easy to get at and playing second position will become 2nd nature with music in keys with two or more sharps. The keys of G, C or any with flats can be played in first position with the exception of the Bb, Eb, Ab on the 6th fret and A,D,G on the fifth fret. I'll bet you get it in a week of trying.