The Celtic Prayer posted below is some of the best words I know. It can be modified if the occassion does not call for such explicit Christian references.
The words of Ecclesiastes might also suit,if only because they so perfectly describe the situation of mourning; though again, they might require modification:
Remember [your Creator] in the day when the guardians of the house become unsteady, and the strong men stoop, when the women grinding the meal cease work because they are few, and those who look through the windows can see no longer, when the street doors are shut, when the sound of the mill fades, when the chirping of the sparrow grows faint and the song-birds fall silent; when people are afraid of a steep place and the street is full of terrors, when the blossom whitens on the almond tree and the locust can only crawl and the caper-buds no longer give zest. For mortals depart to their everlasting home, and the mourners go about the street.You have, of course, Auden's "Funeral Blues"
Remember your Creator before the silver cord is snapped and the golden bowl is broken, before the pitcher is shattered at the spring and the wheel broken at the well, before the dust returns to the earth as it began and the spirit returns to God who gave it.--Ecclesiastes 12:3-7
Stop all the clocks, cut off the telephone,The only other offering I can make is the Psalms. I might make a bold suggestion, on the use of Psalm 23 ("The Lord is My Shepherd)", and that is to preface it with some of Psalm 22:
Prevent the dog from barking with a juicy bone,
Silence the pianos and with muffled drum
Bring out the coffin, let the mourners come.
Let aeroplanes circle moaning overhead
Scribbling on the sky the message He Is Dead,
Put crepe bows round the white necks of the public doves,
Let the traffic policemen wear black cotton gloves.
He was my North, my South, my East and West,
My working week and my Sunday rest,
My noon, my midnight, my talk, my song;
I thought that love would last for ever; I was wrong.
The stars are not wanted now: put out every one;
Pack up the moon and dismantle the sun;
Pour away the ocean and sweep up the wood,
For nothing now can ever come to any good.
W. H. Auden
My God, my God, why have you forsaken me?That, or some portion, could be followed by the comforts of Psalm 23.
Why are thou so far from helping me, from the words of my groaning?
2 O my God, I cry by day, but you do not answer;
and by night, but find no rest.
14 I am poured out like water,
and all my bones are out of joint.
My heart is like wax
is is melted in my breast;
15 My mouth is as dried up like a potsherd,
and my tongue sticks to my jaws;
you lay me in the dust of death.
19 But you, O Lord, do not be far away!
O my help, come quickly to my aid!
20 Deliver my soul from the sword;
my darling from the power of the dog.
21 Save me from the lion's mouth!
I hope this helps.