Many people play the following lick (first two measures) and use it 
when playing against a G or D7 chord. 


I---0---X-------0-----------0-------I-------0-------0-----------0-------I
I---------2-3-----------3-----------I-2-3-----2-3-----------------------I
I-----------------------------------I---------------------3-2-----------I
I-----------------------------------I-----------------------------------I
I-------------------0-----------0---I-------------------0-----------0---I
          H.O.                        H.O.    H.O.         Sl.


I-------------------0---------------I-----------------------------------I
I-----------------------------------I-----------------------------------I
I---0---X------2-3------0-----------I---0---X---X---X---X---X---X---X---I
I--------------------------- 2-0 ---I-----------------------------------I
I-----------0-----------------------I-----------------------------------I
                Sl.          P.O.       



By changing the 1st string open (D) to the 4th string open (D), you get 
a more dramatic sound that tells people "I'm DEFINITELY playing a D7 
lick now!". And by changing the 1st string open (D) to the 1st string at 
the 4th fret (F#), this further broadens the sound of your D7 chord 


I-------X-------4-----------4-------I-------4-------4-----------4-------I
I---------2-3-----------3-----------I-2-3-----2-3-----------------------I
I-----------------------------------I---------------------3-2-----------I
I---0-------------------------------I-----------------------------------I
I-------------------0-----------0---I-------------------0-----------0---I
          H.O.                        H.O.    H.O.         Sl.



I-------------------0---------------I-----------------------------------I
I-----------------------------------I-----------------------------------I
I---0---X------2-3------0-----------I---0---X---X---X---X---X---X---X---I
I--------------------------- 2-0 ---I-----------------------------------I
I-----------0-----------------------I-----------------------------------I
                Sl.          P.O.