Here's some info that helped me personally... (taken from Japanese for Everyone textbook):
(1) Always pronounce vowel sounds clearly and the same length. Never let them sound like "uh" as in mother.
(2) Do not round your lips when pronouncing [ U ] and [ o ] as you do in English. Practice saying these sounds, holding your lips apart with your fingers.
(3) When [ I ] and [ U ] come between any pair of voiceless sounds (p,t,ts,ch,k,s,sh,h,or f), they tend to be whispered.
And... in particular, it had an explanation on "combined vowels":
"Often two or more vowels come together in series. Remember to pronounce each vowel with its proper sound and length. When two or more of the same vowels come together, simply hold the sound for the proper number of beats."
"There are two exceptions to the above. When pronouncing the vowel sequences [ei] and [ou], modern speakers of japanese often don't bother to give the 2nd sound its proper value. Instead, [ei] sounds like [ee], and [ou] sounds like [oo]. Consequently, we have romanized all such sequences as [ee] and [oo] in this book."
Anyways my 2 cents is... I think it's best to not think of them as "combined" vowels- they're not combined! Each vowel is a separate syllable, and shouldn't be "blurred" with the vowel next to it, or it will sound sloppy to Japanese people. Try saying "mae" for example, really slowly... It's just "maaaaa-ehhhhhh"... Now, say it a bit faster til you get the hang of it. At a fast speed, it might sound like "my", but it's not... Maybe for practicing these kinds of words it would help if you tap as you say each syllable.
Another example... I was talking to a Japanese woman a couple of months ago about the anime, "Nausicaa of the Valley of the wind". (naushikaa). When I said it, I said "now-sick-uh" (anglicized), but she said "Na-U-shi-kaa". That's when I really realized the difference between combining vowels and saying them separately. When she said it, I noticed that the "nau" seemed "dragged out" compared to mine, because she was saying it as 2 syllables but I was just saying "now". Also her "nau" had a definite "U" sound at the end...
Hope that helps