I have lived in Russia (St. Petersburg) for a couple of years in the past. I know the poeple and country well. Theft is no greater than in other parts of the world and I would simply recommend all the precautions you would take otherwise. Bloody great big visable chain and alarm and park where you know it's safe. Moscow is a huge unwelcoming city but worth going to see at least once. The people are not as forthcoming as those in other cities but will help you. In my experience the younger generation speak some English in many cases while the older generation speak some German. I would recommend you learn a few phrases in Russian though. Note since Moscow became the capital again in 1918 it has been the showcase of Russia. Most the money and industry in the country is in and around Moscow and it shows. Only the best of everything in Moscow. The roads are usually perfect, the city looks better than most others. You'll find it big and very expensive. Go careful with your money. It'll disappear faster than a rabbit down a hole.
Many older buildings (and not many of those left in Moscow) have courtyards inside the buildings where one can park cars and bikes. I would take a cover to put over it while parked inside a courtyard to stop things disappearing from it as in any part of the world. Sometimes opportunist theft by young people is fun in this country and in others, so as I said, just take what precautions you would usually.
As for pickpockets and the like, yes. Take care of this. At the risk of sounding racist, please avoid the Tajik gypsies in the cities. They often travel in groups/families and are recognisable by their brightly coloured clothing. They travel in groups to distract and steal from you. Of course they are not alone. But be cautious always. My advise is to blend in. Carry some things in a plastic carier bag and NEVER NEVER carry things in bags labeled 'Canon' or other brands. Wrap any camera around your wrist a couple of times with the strap and otherwise you'll be fine.
Be prepared for some lunatic driving in Poland and Russia, especially in Poland. No offence intended but Polish car drivers scare me on the main roads. They have a habit of heading towards you full speed in the centre of the road (which is the overtake lane) even if you are doing the same and you have to try to move in front of the lorry on your right before he/she hits you. Scary stuff!
Just outside Posdam in Poland there is a new toll-road on the way to Warsaw. Use it!! My experience is that it's very empty and cheap. One can travel a great distance in a short time. The condition of the roads is different in the east and west of Poland. Those connecting with cities like Berlin in the west tend to be in a better state than those in the east. Also be aware of horses on the road in the eastern part where they are used on farms more often that in the western half.
In Latvia, drive SLOW! There are enforced speed limits everywhere and police with laser/radar guns everywhere hiding in copses and sideroads. be careful with the speed. This means inside AND outside of the city. It may take forever to cross Latvia on empty country roads but you speed up and that'll be the time the police are waiting with the speed guns. Don't risk it.
Also, from Poland onwards be aware of 'border zones'. These are areas just before and after a border where an enforced slow speed limit applies. I have been caught by over-zealous police crossing the Polish - Lithuanian border and told to pay a fine which was clearly false. Some haggling reduced it down but there was no receipt and the money disappeared into the police pockets I am sure. Be careful and hide all your money except a small amount left in your wallet.
You'll also need to buy insurance I expect when in Lithuania. The 'green card' insuarance doesn't seem to be excepted. You can buy for all three Baltic countries or one at a time. It's not expensive and although your insurance may cover you I found it doesn't help trying to explain that to them.
The road you're taking across Latvia I have travelled. It might be worth heading to one of the border crossings on a less busy road into Russia than a main one but my friend and I were stuck on the border for 4 hours once while all was checked and papers stamped. This is where having your bike insurance and travel insurance in advance helps and I cannot stress this hard enough. The Russians love their paperwork. They always have going back centuries. It's not an old Soviet thing. Papers must be stamped and in the correct order. If you have someone who speaks Russian with you who can help then this would be useful because chances are the border guards do not or will not and will not go out of their way to help you. They love being unhelpful in my opinion. You might be lucky though. Everytime I have been back to Russia, something changes except the roads though. Infrastructure is improving but still too many roads with holes one can sink foundation pillars into.
Also note that the Russian Traffic Police have road stops on roads heading into and outside of cities. Slow down here but not so much to attract attention to yourself and give the police all the paperwork you have, bike insurance and visa etc. in Russian, if they stop you and request it. They are usually polite and salute first but not all are honest and be prepared for the odd bribe now and then. If you're friendly with them they will be with you. Again, never carry all your money in your wallet. Just a little bit. I usually travel with a thousand roubles max and then rest in plastic. There's about 50 roubles to the pound presently and has been for years. The rouble is very very stable.
The speed limit in Russia in theory is enforced but in practice, watch out for lunatics in big cars. The bigger the car, the bigger the arsehole inside is even truer in Russia as other countries. They 'own' the road and won't take too kindly to being reminded they do not. They come up at you are great spread, overtake on blind corners and generally act like, well idiots. I have seen Russian police with radar guns but not often. Best to judge the road conditions as your think.
When it comes to the border between Ukraine and Moldova be aware that there this area comes under the Trans-Dniester administration. They beleive themselves to be a seperate country from Moldova (and a very very old fashioned Soviet one too). I don't know how this will work with your passport. I don't beleive EU citizens need Moldovan visas but Trans-Dniester? You might want to check with the Moldovan Consular division in London there.
Again I wish I was going with you. Presently not working and have been looking for a trip to do like this for a while.
Marky