As many know, 1.6 has been delayed, but there are several things you can do in preparation to ensure you get your maximum return and have the lowest chance of having issues:
1. Take anything out of your mail and store it on a character. Your mailbox is not a bank, as ZOS has pointed out to me in a few trouble tickets, and letting things sit in there makes it more likely it will be deleted over time or when a major patch happens. Don't be surprised if support refuses to restore your legendary items lost
2. Make sure you have 10 less items from a full inventory if you use a purchased pack horse. When we change to 1.6 your horses will be made into one mount With their trained stats converted. Stats from purchase, like the 10+ from a pack horse, will not. Others had random items deleted on PTS, so keep your items 10 below the max!
3. Make your vr5 food now or sell off ingredients. A lot of ingredients will be set to old when 1.6 hits. While we can't be certain some will remain that way after feedback, you can count on your food to be upgraded, so save yourself dome worry. Some notable items are things like "imperial _______" which cost 63 from grocers today will sell for 10 when set yo old. Uncommon pepper, used in consummate honey brittle, will go old. Uncommon oats, used in consummate sweetrolls, will now be used in drinks. Here is a post with more details done by @Sneak_Thief.
4. Screenshot or youtube your achievements. If you pride yourself as an achievement hunter and don't want to lose a particular dye or title, record it incase some get lost. Around 1.3/1.4 older players will remember some achievements were lost randomly, so Screenshot it to give support proof of your deeds!
5. Train 3 horses in one specific stat. As mentioned earlier the top 3 trained stats will be combined into a single riding skill, so you can train each Stat 3x faster today than tomorrow, when you can only train one at a time. This applies to mostly imperial edition players at this point, since only the one gold horse would be cost effective.
6. Store quests if you're getting your max CP on conversation. Storing quests simply mean to complete quests up till the final turning stage but not completing it. If your getting your max 70 champion points, because you have a vr14 or multiple VR characters, then storing these will give you a nice boost when 1.6 comes out. Even if all you do is grind, get some ready! The most exp will probably be Cyrodiil quests, craglorn quests, and some of the Cadwell quests. Remember you can keep 25.
7. If you're not getting max CP on conversion, keep leveling! If you won't get your max CP points get all the exp you can on veteran level chars. The more you do now will get covered to a lot more when 1.6 hits. Since the grind spots are also getting nerfed you will also probably hit a leveling curve as well.
8. Store golden undaunted keys. This can be a pain, because they can't in the bank, but save these till 1.6. The drop rate on shoulders is increasing and the sturdy versions are gone.
9. Hold off on buying pvp coffers. There are VR 13 and vr14 versions coming in 1.6
10. Farm a few purple motifs and recipes. you don't know how hard farming will be, so get some and store some if you can. Prices will also probably go up with the buy to play release
11. Stock up on the basics for crafting. certain basic ingredients and motif essentials will now be in owned drawers, so stock up on a few before it becomes illegal.
12. Stock up on raw materials. Doing writs will give more rare materials when 1.6 comes out. Hirelings will give fewer gold mats, so save up the raw material and get ready to fill out more work orders. There is a debate if refining is giving the proper amount of rare drops, as it was to be increased in 1.6 but it's much lower on PTS. It's a gamble to refine or
not refine them right now, so I recommend to wait to see if it's fixed. other alchemy ingrediants like dragonthrone and blessed thistle may also increase in price.
13. Save some different gear. 1.6 changes a lot, so stop trashing your heavy armor or medium armor drops, some of that may be more powerful now for your play style. Also some potions like stamina may be more viable.
14. Prepare to sneak and lockpick. Thieving is fun, save some lockpick and practice your sneaking. If you're not as sneaky as a khajiit or Nightblade look into the nightshade or nightmothers embrace dropped sets. These will reduce your detection radius. Read this post on legerdemain for more thriving details done by @DeLindsay.
15. Read the 1.6 patch notes! Things are constantly changing so hop over to the PTS forums to see what else has changed!
16. A lot of skills and passives are changing, review your class and plan a build A lot of changes are coming, after you read the patch notes plan to see if you're going to make a new character or modify your current. Get any screenshots of your current build or use PTS while its up!