In French, "s" at the end of a word when it's in plural makes no sound.. So by habit, a lot of french speakers, don't pronounce it.
You require a lot of practice and focus as you speak. One way to improve is by teaching it to someone.

I realized that the best way of learning is by teaching someone else. You review as you teach it.

In the particular case of Spanish, my very unprofessional observations are that:

1) a final "s" is omitted or almost silent in many national varieties of Spanish. I can't give you a list but it seems very characteristic of El Salvador.

2) I asked a native Spanish speaker and Spanish teacher why it is so common for Spanish speakers to mispronounce words like "speak" or "station" by adding an initial "e" ("espeak," "estation"), when there are many Spanish words that begin with an "s" sound.

(Obviously, adding a phoneme to the beginning of a word can't be "lazy lips!")

He said that Spanish has a very strong syllable structure, and that syllables have to contain a vowel... and therefore, in Spanish, you can only have two consecutive consonants in the middle of the word, not at the beginning or end. (I haven't checked on whether this is rigorously true).

If this is correct, then we should expect to hear native Spanish speakers having no trouble pluralizing words if the singular ends with a vowel, but only when it ends with a consonant.

Bill, that's a good question.  When the s comes in the middle of the word (as opposed to the plural s), then there are not good rules and you have to learn each word individually.  You have identified the problem with "loose" and "lose".  Though the plurals of both verbs end in -iz, loose has an /s/ and lose has a /z/. 

Some words which rhyme with loose tend to be spelt with -ui or -oo e.g. juice, sluice, goose, moose.  But then there is choose and bruise which have a /z/ and so it is hard to find reliable patterns.

Newspaper can have an /s/ or /z/