(Note: The following will be most helpful to American/Canadian singers.  I have not had experience working with English speaking singers from other areas, so this advice may or may not apply to you.)

This is one of the easiest things to help clean up your sound if you are aware of it.  The English language has many pitfalls for the unaware singer.  The biggest pitfalls are dip thongs and internal and ending l's and r's.

A dip thong is a vowel sound that changes. The most common are the ah-ee combination (I, like, my, etc.), ah-oo combination (how, about, house, etc.) and the oaw-ee combination (boy,toy, etc.)   There are others. The main thing is to be aware of these dip thongs and have a plan of action for singing them based on the style of singing.  The basic rule of thumb is to sing on the first half of the dipthong for the longest period and only singing the later vowel sound at the end, as one would a constanant. This will give you a more open sound immediately. However, some vocal styles sing dipthongs differently.  Country singers often "twang" the dipthongs, and jazz singers will often drag them out; in either case, the dipthongs are emphasized and exaggerated.  Study the singers in the style you are trying to emulate to identify your strategy.

Internal and ending l's and r's pose difficulty because so many of us tend to anticipate these consonants by tensing out tongue.  Like I stated previously, tension is always to be avoided.  What to do?  With the l's, stay on the vowel as long as possible and let your tongue make the l as independently as possible. (practicing la,la,la, etc. without jaw movement is beneficial.)  As for the r's, try singing through them as a British person would.(Example: here would be pronounced, heah)  I know, this seems really effected but if you listen carefully to other singers, I guaranty that the majority will sing R's in this manner.  For an R that is initiating a word or syllable,the technique for L's also applies