1. "R" is only pronounced before vowels, either in the same word or in the following word (in which case you use the R to"link" the words). Do NOT pronounce any R's before consonants ("worst", "party") or at the end of words ("car", "dear").
2. Articulation is clearer and more precise than in most American dialects, which tend to get a bit sloppy and lazy-mouthed. Medial 'T's really sound like T's and not D's ("better", ..seventy"). Final consonants are clear and strong (.'gifts", "sounds", "things"). Again, though, don't overemphasize speak clearly but remain relaxed.
3. Prefixes and suffixes are always weak (unstressed) and usually take the vowel sound "ih".
--endings such as -y become "ih" with no stress at all: (really, silly, happy)
--endings such as -ary, -ory receive no stress on either syllable and often lose the second-to-last syllable entirely: (military, ordinary, ceremony, solitary)
--prefixes such as be-,re-,pre- become "bih", "rih" and are unstressed (believe, rehearse)
4. High front vowels such as EE, ih, eh, 1, A tend to be "brighter" -lips more smiling and jaw less open than in American dialects
(he, his, remember, presence, switch, still, will, high, hate, fright, spider, take)
5. "AW' sounds are much "darker" -lips more rounded, more forward
(call, awful, thought, draw, brought, water, daughter, walk, talk)
6. The British "OH" is made without rounding lips, sounds like eh-oo
(no, go, own, cold, phone, nose, broken, suppose)
7. "00" and "oo" as in "food" and"foot" are made with more rounded lips
(00-true, threw, who) (oo-would, could, put, good, woman, look)
8. "UH" of "cup" is more open, tongue relaxed and flat, feels "wider" in the mouth
(cup, love, one, once, wonderful, bloody, duck, nothing)
9. The short "a" sound of "cat" is more open, tongue relaxed and flatter than American sound
("Harry" vs. "hairy", can, thank, carry, fancy, angry, chat)
10. There is a special short British "o" sound that is not found in American dialects. This sound is very short with lips slightly rounded, slightly "darker" than the American short "o"
(not, hot, obviously, job, honest, Tom, soft, long, strong, forgotten, knowledge, sorry)